Piers Anthony does these occasionally. One story he wrote was basically a Take That! explaining why the sci-fi publishing business was worthless (Anthony having struggled against it for quite some time before learning the tricks of the trade). One supposes that subjectivity enters in over where the line is drawn between Author Tract, Author Filibuster, and Author Appeal where his other books fall, though he's never been very shy about making his ideas on sexuality (and the ages at which people take notice of it), body modesty, and other things an important plot element of his stories.
Ray Bradbury uses his story "The Toynbee Convector" (title story of his mid-1980s collection) to rail against his society's defeatism and negativism at the time. It is out of character for Bradbury but works if you view the big lie of the story as representing the writer's art. In that view, Bradbury is just saying how he hopes his writing will influence the "real world" (or bragging that it has had that effect).
A lot of Meg Cabot's books, especially her YA novels. It was especially apparent in Ready or Not, where Ms. Cabot literally stopped the narrative to rant against the abstinence movement. Her other books contain some amounts of similar commentary.
Orson Scott Card's Empire, where the characters will pause during the action to explain exactly why sweeping demonizations of the views of others are destructive. Part of it comes from the ridiculous premise—he was hired to write the backstory for a video game about a second American Civil War taking place 20 Minutes into the Future, with the opposing sides being strawman versions of the Democrats and Republicans.
Towards the end, the Ender's Shadow series also features numerous lectures from widely disparate characters on how the only way to really be a part of the human race is to have babies, culminating in one Battle-school grad stopping her troops in the middle of a battle and telling them to go home and procreate.
More recently his novella Hamlet's Father, a retelling of Hamlet, has been accused of this. In it, Hamlet gets portrayed as staunchly Christian with a firm belief in the afterlife, very different from his doubts about this in the play, but in keeping with the author's beliefs. These revisions would be controversial enough themselves, but it's also shown that his father was a predatory pedophile who sexually abused Hamlet and many other male characters. In fact, his father was not killed by Claudius, but Horatio, in revenge for this abuse. Worse, it's implied this turned Hamlet and the other victims gay. Card has disputed this view, but it agrees with his publicly stated theory on what causes homosexuality.
Eoin Colfer is often fairly pro-green in Artemis Fowl and The Supernaturalist, but he really dials it up in The Fowl Twins. In the first book, it's claimed out of nowhere that no fairies are naturally aggressive, and the ones who are have simply been poisoned by human pollution.
Michael Crichton's books sometimes veer into this. In many of his books, he includes a little author's note at the beginning about the real-world issues the book explores, along with an Author Filibuster or two somewhere in said book. State of Fear was an anti-global warming opinion piece veiled as a work of fiction. He devoted the last 50 pages of the book to a huge author's note, complete with bibliography and list of cited works. The story itself even has citations, and most of the villains are strawmen environmentalists.
Thriller author and former US Navy Captain PT Deutermann uses his political/military thrillers to air his opinions about military bureaucracy, politicking by senior military leadership (especially the Navy), social engineering and other military-related issues. Especially evident in Scorpion in the Sea (Head-in-the-Sand Management by senior naval officials), The Edge of Honor (the draft, lowering of standards), Official Privilege (race issues in the military, too much power in the hands of admiral executive assistants), Darkside (social engineering, lowered standards, and hypocritical senior leadership at the Naval Academy), Cold Frame (morality of drone warfare against terrorists).
Philip K. Dick put varying amounts of his own beliefs into his stories, but his short story "The Pre-Persons" is very blatantly his personal, heavily emotional response to Roe v. Wade, set in a world where pro-choice activists have legalized "abortion" of children up to age 12. His mouthpiece characters claim abortion is all about powerful people deliberately picking on the helpless, or a certain kind of woman getting off on destroying men and children. He even depicts one woman wanting to get pregnant because she thinks an abortion would be fun and a turn-on.
This trope was Charles Dickens' stock in trade. All his works are morality plays meant to drive home his socialist (or at least social-democratic) ideals. In A Christmas Carol, Ebenezer Scrooge rails that the poor are lazy and inferior and deserve to die, on scientific principle, and then an innocent child almost does. In David Copperfield, Nicholas Nickleby, and Oliver Twist, more innocent children are mercilessly abused, either by predators that society chooses to do nothing about or by the very institutions of that society. In Little Dorrit, citizens are reduced to professional beggars by the debtors' prison system. In American Notes, he praises American attempts to reform asylums and condemns slavery. And the list goes on.
He hoped to convey a new way to understand religion through exemplifying the themes of guilt and free will in writing The Brothers Karamazov. This can be seen in what many critics call the pivotal chapters of the book, which include the parable called ''The Grand Inquisitor''. The way in which events play out conform with the Elder Zosima's idea expressed throughout "everyone is guilty for all and before all."
Notes from Underground is arguably an Author Tract; it highlights the societal chaos brought about by the then-fashionable, and highly depressing, trend towards rational nihilism ("nihilists" in Russia also meant radicals who wanted to violently "remake" society by destroying all the existing institutions).
Crime and Punishment is an Author Tract in the same vein, with the main character being a cruel nihilist who kills an elderly loan shark to rob her of the money he needs for university, justifying it on the grounds that "great men" such as Cesare Borgia showed no qualms about doing such things in pursuit of their goals. He winds up repenting and becoming an Orthodox Christian. Not surprisingly, this was Dostoevsky's religion.
The Czech children's writer and Scout leader Jaroslav Foglar almost always wrote Author Tracts in one way or another, with morally upstanding characters, characters tackling moral dilemmas that he felt he ought to address, from his own experience working with children, or simply describing the way he thought child collectives should function. He was very good at making it entertaining, but he also definitely suffered from tendencies to use too much pathos and too much of a didactive voice. However, when you know Foglar mostly as the creator of the completely unironic Incorruptible Pure Pureness character of Mirek Dušín, it can then come as quite a surprise to read, say, Tábor smůly and realise Foglar was equally capable of tongue-in-cheek...
John Grisham's books often feature this trope, targeting big business and/or conservative views.
The Confession: The book attacks the death penalty by constructing a miscarriage of justice where the pro-death penalty side is all grossly negligent and unlikable, in contrast to the anti-death penalty side. To top it off, once the message is thoroughly beaten through you, Grisham decides to dedicate a few pages to having a character rail against the death penalty.
The Appeal features a long discourse on the need for an independent judiciary, how ads manipulate the truth, and how often big businesses will hide behind certain causes as an excuse to manipulate tort law to be more favorable. Including having a train of accidents hit the winning election candidate to get him to try and convert, but he stays bought.
Arthur Hailey's novels often go into Author Tract territory, as the author has one or another of his characters expatiate on a particular failing of the business he is examining in the current book.
Airport goes into a lot of detail about aviation safety, how people who complain about airport noise are in fact sometimes deluded by real-estate promoters looking to make a buck, and the evils of "flight insurance" (a type of life insurance which, at the time the novel was published, could be purchased by passengers worried about whether they would survive the flight).
The Moneychangers has a recurring character to filibuster about how Gold is Good. Given that he's a pundit with his own popular newsletter and is married to one of the secondary characters, and the book is about banking, it kind of makes sense. Then, after the 'real' ending, the US establishes a gold-backed dollar, and we are treated to the full text of one of said pundit's newsletters. Guess what it's about? The book ends with the protagonist putting the newsletter down and reflecting how wise said pundit is.
This makes even less sense in Overload, a novel about a power company, when the President establishes a gold-backed dollar. The protagonist, a power company spokesman, promptly comes up with a perfect comment about the dangers of America's dependence on foreign oil, as requested by the reporter who presented the story to him so she could get a soundbite.
Starship Troopers is an Author Tract, all right. Robert A. Heinlein wrote it in protest of America signing a nuclear treaty with Russia — whose leadership he did not believe would keep nuclear treaties.
A large part of Heinlein's Stranger in a Strange Land revolves around nudism and polyamory, both of which Heinlein practiced in his real life (For Us, The Living: A Comedy of Customs , a lost early Heinlein manuscript which was first published in 2003, contains similar themes). Indeed, his works can largely be divided into pre-Stranger and post-Stranger, with the latter showing far more evidence of this. There's also a greater-than-average amount of incest, including a mention that in his distant future it's genetically safer in some cases for a woman to bear her brother's children than an unrelated man's — a couple's decision to have children together (or not) is based purely on their gene scans, not on consanguinity. Not that that necessarily stops them from marrying; there's a reference to a happily married couple who are raising seven children, "four his, three hers, none theirs," using donor sperm for hers and donor eggs for his because the genetic risks of having children together were too great. Heinlein was probably unaware of the Westermarck Effect, or he would have been less sanguine about the possibility of genetic scans completely replacing the incest taboo as society's method of minimizing pregnancies and births marred by reinforced harmful recessive genes.
In Battlefield Earth, psychiatry is what caused the evil space overlords to turn from their generally happy live-and-let-live prior existence, into amoral Planet Looters who regularly commit planetary genocide just so nobody will get in the way of their mining operations.
Psychiatry is also the most evil force in Mission Earth, to the extent that every single antagonist is either a supporter of the profession or a practitioner or exporting it off-world or using it to take over the world. It doesn't help that almost every character is a Straw Character. For example, the evil Psychlos. This isn't a play on 'psycho' — it's a reference to psychologists, who are considered evil in Scientology doctrine.
His earlier work Masters of Sleep promotes Dianetics and features as a villain a mad psychiatrist, Doctor Dyhard, who persists in rejecting Dianetics after all his abler colleagues have accepted it, and believes in prefrontal lobotomies for everyone.
Other common targets for Hubbard's ire include journalists, federal investigators, bankers, elected officials, policemen, doctors, college professors, and modern art. The first two had conducted investigations of Scientology, earning them his animus.
Most books by Dean Koontz has at least a few rants about the many things that Koontz considers to be wrong with the world, which while never explicitly tied to one political direction or another usually maps well onto the dumbest extremes of liberalism. Sometimes this ties into the themes and plots of the novel — for instance, Dark Rivers of the Heart is explicitly about governmental overreach and the dangers of people trying to use the government's power to create utopia, with a helpful afterword where the author explains exactly what he thinks on the subject — but most of the time the deranged anarchists, anti-intellectual poets and welfare cheats just seem to be there to highlight how wonderful the protagonists and their implicitly-conservative values are in comparison. It's quite common for some characters to just sometimes start thinking about how evil abortion is, how ridiculous the idea of evolution is or how anything other than Black-and-White Morality is just an excuse to do evil, regardless of whether or not it has any relevance to the situaiton at hand.
The Great Divorce, is an allegory about how everyone in Hell could leave and go to heaven at any time, if they were willing to give up whatever sinful obsession it is that they are most holding on to and fully accept God instead. The majority of the examples in the book are ultimately unwilling to do this, though a few do.
The Pilgrim's Regress, which Lewis wrote this as a deliberate allegory when he thought his path to conversion was typical. He later found out it wasn't.
The Screwtape Letters is all about various ways that demons tempt a person into damnation. Lewis notes in the introduction that, being a demon, Screwtape is an Unreliable Narrator.
Ninety-nine percent of everything that John Milton wrote (including, tautologically, his political tracts) were author tracts.
A lot of Larry Niven and Jerry Pournelle's collaborative work have a message that technology and science are good, religion and tree-hugging extremists who hate technology are bad.
Pournelle's CoDominium backstory is one huge author tract, mostly in regards to socialism ruining the economy and society, though the tract is mostly absent from the actual meat of the novels.
Ironically, the author tract was greatly reduced when Niven and Pournelle collaborated on The Mote in God's Eye, set in the CoDominium universe, albeit several hundred years later.
Lucifer's Hammer uses the Straw Character device to represent feminism, New Age religion, and environmentalism. These straw characters die shameful deaths during the Apocalypse How plot.
George Orwell, a staunch democratic socialist, spent a lot of effort to warn others about the dangers of surrendering to dictatorships:
Nineteen Eighty-Four is an Author Tract based on Orwell's vision of the Soviet Union and on what rampant ideological totalitarianism can lead to.
Petrarch's unpublished final work, a poem on Scipio Africanus, was full of long Author Filibusters on how Ancient Rome was better than everything ever. Technically, this is true of all of Petrarch's work, and indeed, most things that were written during The Renaissance, but he exaggerated the cultural inferiority complex. There's also apparently a fictitious bit where Scipio goes to see a fortuneteller, who speaks of a dark time when poetry will die out and only a man named Petrarch will be able to save it.
Even Edgar Allan Poe wasn't immune to this, though, to either his credit or his fault, he restricted it to philosophy.
The Imp of the Perverse is entirely about his idea of a previously uncredited motivating force behind people's actions.
There's also a passage of about a page or so in The Purloined Letter in which the protagonist, C. Auguste Dupin, explains why mathematicians aren't very good at reasoning. This is tangentially related to the story, but one does wonder if it needed to be explored in such detail.
Ayn Rand wrote several novels expounding of the virtues of her personal philosophy, Objectivism, culminating in her Magnum Opus, the DoorstopperAtlas Shrugged, which features theAuthor Filibuster (actually only the longest of several in the book), lasting dozens of pages on end. Of course, like George Orwell, Rand never pretended her books were anything but author tracts.
John Ringo (a self-described Tea Party Republican) does this fairly regularly, more so as time goes on.
The Last Centurion, written in a blog-type format, takes issue with various issues held dear by liberals, including universal health care, interracial relations, and "government knows best" attitudes.
In Through the Looking Glass, a grandmother ponders why her local Democrats can't be both liberal and patriotic, though this is also a first-person perspective. Later, it turns out various Terrorists and Insurgent groups tried to use captured aliens as a bioweapon, which escaped of course and butchered most of the Islamic fundamentalist movement in the Middle East. It's a good thing for the characters.
He wrote Ghost as this deliberately. He never intended to publish it, but it got published due to pressure from the fans, much to his chagrin — he has described it as "the wanker piece" and "the spewings of my id." To give you an idea, the main character pursues kidnapper terrorists to the Middle East, where he kills them all, coaches a group of naked coeds through a siege (while renaming them, because he can't be bothered to learn their names), kills Osama bin Laden and mails his head to the President in a bucket, buys a yacht with the reward money, has kinky bondage sex with some of the coeds and converts them to Republicanism. Later volumes in the Paladin of Shadows series, which tone down some of the more extreme elements of the first book, take aim at extremist Muslims, bureaucrats, and assorted other issues that bother him.
Ringo uses Troy Rising to lay into some of his usual Pet Peeves: Pacifism is dumb, the military is extremely important, Nepotism is the bane of humanity, liberalism is evil, the Mainstream Media cannot be trusted. New(ish) ones include "space is really dangerous", "Science Is Good", and "maintenance is very important." The storyline features most "big city liberals" being wiped out when the cities are destroyed, most Muslims (at least, the ones not smart enough to be "modern Westernized Muslims") being wiped out by an alien plague, the free market being enforced, and most Hispanic males being shown to be a pack of ignorant macho yahoos.
And lastly, he’s also best known for coining the now infamous term, “Get woke, go broke”.
Hybrids spends a lot of time talking about how evil human males are, and how they've done nothing but bring evil into the world. Plus, when religious/mystical belief disappears it's described as causing many good things to occur, and Neanderthals (who are atheists universally) are in many ways better than humanity. This one is milder however. Hominids in the same series, along with some material produced to promote it, includes many arguments about the evils of privacy.
It becomes pretty obvious what Sawyer thinks about various issues across his novels (e.g. atheism, religion), and this even extends to his pet peeves, such as how January 1, 2000 wasn't the real new millennium given that there was no year zero — rather, it's January 1, 2001. Quantum Night seems pretty heavy-handed against the US right wing too. It's hard to imagine even the most hardline Republican in the US ever invading Canada or abolishing illegal aliens' human rights.
The Downloaded is a massive author tract on how overreliance on technology is going to doom us all. He also doesn't mince words on anyone who doesn't mask up and get vaccinated during a pandemic.
Percy Bysshe Shelley was very vocal about his views and was not afraid to express them in his works. To name some examples, Zastrozzi outlines Shelley's atheism through the mouth of its villain Pietro Zastrozzi. His first major poem, Queen Mab, on top of making attacks on war, religion, the eating of meat, and marriage, came with endnotes in which he expounded on these themes.
Much of Sheri S. Tepper's work reads as thinly disguised, feminist utopianism, particularly The Gate to Women's Country and The Revenants. Beauty paints a rather extreme picture of humankind's 'destruction' of Earth's environment.
British children's author Jean Ure almost always brings up the topic of vegetarianism in her books, and the main characters are often converted to it by the end of the book, such as Cherry in Skinny Melon and Me, Pumpkin from Pumpkin Pie, or the character who is a vegetarian tends to be portrayed as the most sensible person in the novel, like Harmony in The Secret Life of Sally Tomato or Stephanie in Passion Flower.
Jules Verne belonged to a "Heavier-Than-Air" innovation society when he was young, so it's no surprise that one running theme of his major novel about flight, Robur the Conqueror, is a continual deconstruction of the Cool Airship trope. (Of course, since his whole argument was built on hypotheses about future inventions, the novel was also a Fantastic Aesop until practical heavier-than-air crafts were actually invented.)
Cat's Cradle not only talks about how the invention of nuclear weapons was a bad thing, but pretty much says that if we insist on inventing things without thinking first about what they might be used for after we invent them, then we're all doomed (one character has given up science altogether, since he's come to believe that anything he invents will probably be turned into a weapon somehow).
The parts of Slaughterhouse-Five set in Germany during WWII are unquestionably anti-war.
The message of God Bless You, Mr. Rosewater seems to be that society will not fall apart if the wealthy share their money with the poor.
The very first page of Breakfast of Champions begins describing the country in which the characters live (the United States) and all the ways in which it is fucked up.
Galápagos chucks all ambiguity to the winds and spends its length explaining why humans would be better off without the ability to think.
Dennis Wheatley did this in just about every novel he wrote. His books often lapse into political polemic about the seductive evil of left-wing politics and the need for Britain to be governed by a strong benevolent dictator according to the principles of libertarianism and free-market economics. After all, the working classes are too docile and ill-educated, so people like us must shoulder the burden of ruling them, for their own good of course. Alongside the politics, Wheatley also held forth in favor of his religion, a kind of cross between Christianity and Buddhism.
Allen Drury's Advise & Consent is a Government Procedural with a strong right-wing bent: the book's conservatives are flawed but basically virtuous, the liberals are lying, treacherous, Communist-appeasing dupes. The sequels tone this up even further, but The Film of the Book tones it down considerably, opting for Gray-and-Grey Morality with neither side having clean hands.
Americanah by Chimamanda Ngozi Adichie is devoted almost entirely to discussions of race, racism, nationality, and immigration, and most of the scenes in it exist to make a point on one of those topics. Sometimes the author simply eschews the narrative altogether to include "blog posts" on those topics by the protagonist.
Anarchaos by science fiction author Donald E. Westlake comes off as this concerning anarchism, with the main theme being that Anarchy Is Chaos (as the title kind of implies). He posits a world entirely colonized by anarchists, which quickly breaks down into, well, chaos (in the story the world is even named Anarchaos by the anarchists themselves, which seems very unlikely since they usually define "anarchy" as a type of ideal society). His narration directly says anarchism is an idea doomed to fail when put into practice, with the story itself vindicating this. It also really bizarrely portrays the anarchists as moral nihilists who all believe there is nothing wrong with theft, murder or slavery (explaining the social breakdown), a laughable claim to anyone familiar with them (whether or not you'd agree with their ideology overall). This would apply, at most, to a small number of them.
Area 51: Though it works in the story, author Robert Doherty (a true believer in everything his work depicts as true-ancient astronauts and Atlantis, simply to start) puts in a number of slams against mainstream scientists who he portrays as just too dogmatic to accept any evidence which doesn't fit into their theories, even covering it up. To some degree, this comes from the influence of said aliens and government coverups, though a lot is apparently just his personal beef (along with pushing said alternate hypotheses in fiction form). While this does not make the books bad, sometimes it can come off as a bit much (along with insulting to real scientists who reject his views).
August by Bernard Beckett is a philosophic idea about free will (or the lack of) with a two main characters and storyline plastered on top.
The Bill the Warthog series of children's detective stories are meant as biblical metaphors, including a whole book where the author just rips stories from Jesus's parables. Good thing the parables are in the public domain...
Anna Sewell's Black Beauty was originally written as an Author Tract about the abuses suffered by carriage horses in 19th-century England, not as a children's novel.
Jan Guillou digresses in The Bridge Builders on American railroads mistreating Chinese "slaves" building railroads in the American West, and draws the conclusion that Americans, in general, are "the world's most brutal people" — even though the novel has no scenes in the US or China and none of the characters have any connection to the US or China.
Charlie and the Chocolate Factory. Four kids are punished for their flaws, and the one perfect kid inherits a huge chocolate factory. Whilst no one would deny that Veruca Salt's brattishness probably got her what she deserved, gluttony, gum-chewing and TV addiction (particularly the last) are more personal bugbears of Dahl's. (You could argue that these habits are symptoms of the kids' general Jerkass behavior which, as Dahl also points out, is indulged by their parents.)
Christian Nation by Frederic C. Rich is one that speaks against fundamentalist Christianity and their promotion of Dominion Theology, as portrayed through the Alternate History of the United States that follows Sarah Palin becoming its president.
In-Universe in the first book: the school paper's Wacky Dawg comic is cancelled because the author has been using his comic as a mouthpiece to talk to other students.
The Long Haul is heavy with the New Media Are Evil message: Greg, the Unreliable Narrator who's a bad person and is meant to be an anti-role model, says that "electronics are the key to family happiness." Susan bans technology from the road trip, to Greg's dismay.
Since The Long Haul, just about every book since has had at least one anti-technology scene. Sometimes? It seems almost shoehorned in.
No Brainer is essentially one big polemic about Technology, AI, and underfunded public schools.
In Does My Head Look Big in This? by Randa Abdel-Fattah, about a Muslim girl living in Australia who decides to wear a hijab regularly, this occurs a lot. The main character often has speeches about the fact that non-Muslims should just see it as a piece of cloth and not as her whole personality.
Dragon Rider by Cornelia Funke is full of the author's agendas. Every character we are supposed to like is a vegetarian, a pacifist, and will never stop bemoaning mankind's need to put animals in cages even though this theme has cursory relevance to the actual plot, at best. The author places Eastern people high up on a pedestal over Western people to the point of othering them.
Elsie Dinsmore, written by Martha Finley, aimed to teach Children how to be more Christlike by way of the adventures of the title character and was even reprinted under the Life of Faith banner in the 1990s, though with the Values Dissonance toned down significantly. Finley was also the daughter of a Presbyterian minister, just in case you were curious, and most of her other works follow a similar format.
Everything Flows is basically one long statement by Vasily Grossman on Stalinist oppression and the necessity of freedom, with a story to help the digestion.
Executive Orders has President Jack Ryan remaking the U.S. government, after most of its Legislative and Executive branch were killed at the end of Debt of Honor, by a Japanese Airlines 747 crashing into the Capitol Building while Ryan was being sworn in as Vice President.
Fast Food Nation is highly dominated by Eric Schlosser's political and personal views, particularly against fast food companies, big companies in general, industrialization and Republicans while the final chapters are all about defending small and medium food businesses and vegetarian or vegan options.
Joanna Russ's sci-fi novel The Female Man is partly about Alternate Universe versions of the same woman meeting up and getting to know each others' culturesnote one is from the world as we know it, one is from a world where The Great Depression never ended, one is a warrior from a world where men and women are on opposite sides of a war, and the last one is a utopia where men were wiped out by a vaguely defined 'plague' in the distant past, and it's equally about Russ taking every opportunity to espouse how men are keeping her down. It's telling that one of the most detailed passages is that warrior woman literally tearing a man apart with her reinforced steel teeth and claws. It's also implied that the Lady Land utopia is the direct result not of a plague, but of the aforementioned gendercidal war.
The Grapes of Wrath has been a frequent sight on banned-book lists since its publication, because so much of it is extended digressions on why Capitalism Is Bad. Many chapters never mention the main characters or plot; they're just discussing the general state of affairs for Depression-era migrant workers and what the author thinks should be done about it.
John Twelve Hawks wrote The Fourth Realm to alert his readers to an invasion of their privacy.
Hayy ibn Yaqzan, an Arabic novel written by Ibn Tufail in 12th-century Andalusia, is an Ur-Example of this trope. It tells the story of an autodidactic feral child, raised by an animal and living alone on a desert island in the Indian Ocean, who sets out on a journey of philosophical inquiry and self-discovery. Its plot somewhat resembles a more recent best-selling novel, Life of Pi.
Hidden Order by Brad Thor is a novel published in 2013, 12th in the highly successful "Scot Harvath" series. In the book, Harvath has to investigate and unravel an elaborate conspiracy regarding the Federal Reserve Board. It is also a book-length diatribe against the Federal Reserve System, which is blamed for high inflation, devaluing the U.S. dollar, and rigging the system to benefit the top 1% of the wealthiest Americans.
His Dark Materials by Philip Pullman is to teach about atheism and vilify the Catholic Church. Very specifically, it was begun with the direct intention of being an atheist counter-part, and counter-point, to the Narnia series.
The elves of the Inheritance books (Eragon, Eldest, Brisingr, and Inheritance) are atheist vegetarians who impart their wisdom to the main character and the reader, by spending quite a bit of time expounding upon how stupid religion is (particularly to the dwarves). Christopher Paolini denies that this was a representation of his own beliefs, claiming it was simply an attempt to portray various cultures and viewpoints in the series. This became a lot more plausible after the third book. However, in the fourth book Eragon devotes two paragraphs to discussing the stupidity of religion, and in many places, it is hinted that religion is scoffed at by all the main characters except Orik (the dwarf king) and Nasuada (the human queen).
Norman Spinrad's The Iron Dream. An Alternate HistoryAdolf Hitler (who became a writer instead of a politician) writes Lord Of The Swastika, a pulp SF adventure with a plot that mirrors the real-world rise of the Third Reich. It's followed by a review where a scholar heaps praise on Hitler as a brilliant writer of rollicking good adventure stories, and whose only criticism is that he thinks it was a bit implausible for the protagonist to rise to power by creating a rather silly cult of personality and machismo. Naturally, the whole thing is one giant Take That! at the Broken Aesop morality of pulp SF and fantasy stories—and more generally, a Take That! at Utopian fiction in general, satirizing the idea that you can write a book to "prove" your social theory will work in practice. In other words, it's an Author Tract about Author Tracts (specifically saying that if your tract is about the real world—rather than fiction—it's pointless).
The Jakub Wędrowycz stories are written by a conservative author, and it shows sometimes; in one of the stories, the bad guys are radical left-wing ecologists, and in another, the heroes chase away a European Union official.
The Jungle by Upton Sinclair is perhaps one of the most compelling examples we have of an author tract, or rather two tracts — first about the hellishness of the meat-packing industry in Chicago at the beginning of the 20th century, and then a defense of socialism. More literal than the usual author tract, because at first, he had to self-publish. The meatpacking half (based on Sinclair's undercover observations) was so horrifying that it led to nearly-immediate regulation: the Meat Inspection Act, and the Pure Food and Drug Act (which established the FDA). The socialist half made little lasting impact in America, where the burgeoning movement was forcibly shut down by the government but was part of a sweeping movement that radically transformed the politics of Europe and Asia.
Sinclair wrote a lot of books and most of them were author tracts to some extent. He lampoons this In-Universe in his 1942 book Dragon's Teeth. Lanny's friend Rick is so upset by Ramsay MacDonald betraying the Labor Party and entering into a coalition with the Conservatives that he writes a whole play about it called The Dress-Suit Bribe.
King John of Canada by Scott Gardiner, although nominally a political satire, in reality, consists of one Author Filibuster after another against Natives, Quebec Separatists, environmental activists, Saudi Royals, the Asper family, American-style conservatives... in short, everyone that the author doesn't like.
Knowledge Of Angels: The entire book is one of these about theism and atheism, set as a story.
The Land of Mist by Sir Arthur Conan Doyle is a novel-length tract justifying the author's conversion to Spiritualism, including the massive change in the character of ultra-rationalist Professor Challenger, who converts to Spiritualism. There is a suggestion in chapter two that the deaths of "ten million young men" in World War I was "punishment by the Central Intelligence for humanity's laughing at the alleged evidence for life after death".
The Left Behind series of religious novels are overtly based on the authors' premillennial dispensationalist views on the Rapture. Only Christians with their very specific beliefs are shown to be worthy of going to heaven. Like any didactic religious story, the plot is clearly just a vessel to convert the readers or reinforce their already sympathetic views. Helpfully, the two main characters are both Author Avatars, giving the reader a virtually unfiltered look into the authors' actual beliefs and point of view. Slacktivist illuminates many of these beliefs in his page-by-page analysis.
The Last Days Trilogy preaches a pre-wrath Rapture instead of a pre-Tribulation Rapture. The Author Avatar takes to task anyone in the story series who believes in a pre-Tribulation Rapture, claiming that they would most likely be deceived by The Antichrist and take the Mark of the Beast without knowing it.
Looking Backward, an 1887 best-seller set in the year 2000, in which various people go on at length to a Fish out of Temporal Water protagonist about the wonderfulness of their socialist utopia.
Matthew Dickens spends the last hundred pages of the book Magnus telling the reader about his personal views on religious doctrines, evolution, theology, Superman Returns, etc.
Pretty much the entire point of Max Havelaar is to condemn real-life colonialism. In the end, the author comes in and dismisses his characters and says his message (the exploitation in colonial Indonesia) out loud. He admits he didn't care about the style of the book, as long as it was read.
The Maximum Ride novels became one big Green Aesop after book three, particularly The Final Warning.
Nation, by Terry Pratchett, is unusually heavy handed with its themes. If one has read many Pratchett books or has ever listened to him speak on religion, it becomes extremely obvious that the book is almost entirely an Author Tract about humanitarianism, atheism, thought, and the role religion plays in society — as well as colonialism and imperialism. This becomes even more obvious at the end of the book where Pratchett drops all pretense of writing a story and simply has a section that may as well be Terry himself making a speech about humanity.
News from Nowhere, published in 1890, is a riposte to Looking Backward, written by another socialist who favored a more agrarian and less urban socialist paradise.
Noir by K. W. Jeter is a Doorstopper set in a DystopianCyberpunkCrapsack World. The main character is a "Copyright Cop" who spends most of the book discussing how people who infringe copyrights should be dismembered and tortured because, in the Information Age setting of the book, copyright violation is worse than all other crimes. Jeter's personal website indicates that he's against copyright violations himself.
The portrayal of the royal characters as brats and jerkasses while "commoners" like Marie and the Nutcracker became beloved monarchs of the Land of Dolls was meant to attack and subvert the notion that royalty was inherently good and noble and that one needed royal blood to be a good ruler.
It also features a fairly-unsubtle jab at Beauty Equals Goodness with Princess Pirlipat, who's also a deconstruction of the fairy-tale princess tropes.
"Only the Super-Rich Can Save Us!" by Ralph Nader. Yes, that Ralph Nader. Although — consumer advocate that he is — he never pretends that the book is anything other than 'how everything could be so much better if a few rich people got together and implemented my program.'
Pamela, or Virtue Rewarded by Samuel Richardson was a very popular didactic novel to teach young women the importance of feminine virtues, including piety, domesticity, and most importantly chastity. The main character is basically an incarnation of the feminine ideal who repeatedly asserts her virtue against the advances of a rakish suitor.
Henry Fielding was so annoyed by the ideals espoused by Pamela that he wrote two parodies. The first is called Shamela, which ridiculed the concept by having long conversations over the heroine's "vartue", pointing out just how meaningless the word "virtue" is when used in the original. These second is Joseph Andrews, revolving around Pamela's brother and casting him in the role of the young innocent whose virtue is continually besieged.
Howard Zinn's A People's History of the United States has garnered controversy in part because of this trope. An attempt at historical revisionism, and nominally a non-fiction Popular History, it is in fact an Author Tract which claims that American history is little more than a dismal narrative of Working Class Heroes who fight against and often lose toor are duped byelitist villains. For example, Zinn strips the American Revolution of all idealistic motives and claims that the Founding Fathers used itto distract the people from their own economic problems. Rather understandably, the book has been criticized for robbing American history of its richness and nuances and leaving an empty text that boils down to propaganda.
Eugen Richter's Pictures of the Socialistic Future, which has the Straw Character as the viewpoint character who celebrates Germany's slide into Stalinist Communism and saves the Author Avatar for the very end. Interestingly, it was published in 1891 and managed to predict much of the Crapsack World the Soviet bloc would become. The author (1838-1906) was a leading progressive liberal politician of the time of Otto von Bismarck and Wilhelm II.
Astrid Lindgren wrote Pomperipossa in Monismania to make a point about taxes — the point being that it shouldn't be possible to have a marginal tax rate of 102%. Obviously not a very generic point, but it was relevant to when and where she wrote it (since it was written in reaction to finding out that her marginal tax rate was 102%), and it has the advantage of it being something that most people would agree with (it wasn't actually intended to be the case by the taxation system's designers, they'd just failed to anticipate the combination of being self-employed with having a high income).
Living in an age when the legal system was under the domination of the new rich,Mark Twain spent most of The Prince and the Pauper talking about the dangers of a legal system written strictly to benefit the upper class. This is why the film incarnations are so much shorter than the book, because the adaptations cut out a great number of instances of brutal Tudor "justice" that the Prince encounters in his travels: the original book sends a powerful message about the insanity of a judicial system constructed entirely for the benefit of the wealthy.
G. P. Taylor's book Shadowmancer is a heavy-handed attempt to get the reader to convert to Christianity. It's filled with Hollywood Atheists. One of the characters, Raphah, is clearly an author mouthpiece who condemns all things the author dislikes such as witchcraft and coffee.
H. P. Lovecraft's short story "The Silver Key" consists almost entirely of his Author Avatar Randolph Carter, who is exactly like Lovecraft except that his family didn't lose its wealth and prestige, musing about all things wrong with the society. He bashes both religion and science for their obsession with order and structure and declares that dreams are equal to reality and that the only things worth valuing in a meaningless universe are beauty and harmony. The ending implies a romanticized view of suicide, as Carter abandons the Waking World, ironically in perfect opposite to the Aesop he was supposed to have learned in The Dream-Quest of Unknown Kadath.
A minor example from 1632: Almost no down-timers appreciate rock and roll music. Lots of down-timer country music fans. Lots of down-time folk music fans. And it goes without saying that opera and orchestral music are beloved. But... absolutely no down-timer fans of rock and roll. Not even relatively "light" rock and roll like the early Beatles. And the less said about the reaction to rap music the better. Coincidentally, these views happen to mirror the musical tastes of Eric Flint almost precisely.
Sword of Truth is a Door Stopper twelve-book series filled with Author Filibusters from the fourth book on, about the evils of extreme Socialism and of the importance of individual rights and freedom. These themes were always slightly present, but really begin to crop up later in the series: Although the D'Haran Empire under Richard is no less of an autocracy than the Imperial Order, it is one guided by a firm sense of individual liberty championing the idea that every individual should be the best that they can be, and should be free to benefit based on the effort they put in and the skills they possess, and how this benefits society as a whole. By contrast, people under the Imperial Order are living in absolute squalor, and there is a fear of being anything more than mediocre to avoid rising above anyone else and drawing undue attention and punishment from those in power, and how this drags down all of society with it. Goodkind frequently halts the plot of his books so that his main character, Richard, can speechify/rant on Goodkind's views for pages upon pages, sometimes taking up more than two chapters, with all the subtlety of a sledge hammer.
In Rudyard Kipling's "The Three-Decker", he derides the way these writers considered themselves superior to Escapism in the three-volume novel.
Her crews are babes or madmen? Her port is all to make? You're manned by Truth and Science, and you steam for steaming's sake? Well, tinker up your engines — you know your business best — She's taking tired people to the Islands of the Blest!
The Turner Diaries, written under a pseudonym by William Pierce, who was the leader of the neo-Nazi organization National Alliance until his death in 2002. Largely about the evils of liberals and Jews enslaving America, and the actions of the terrorist cell 'The Order' trying to overthrow said strawmen. For a scary note, a scene in which the Order blow up a federal building probably inspired the actions of one of its biggest fans — Timothy McVeigh, the Oklahoma City Bomber. The Order also inspired a Real Life terrorist organization of the same name which is responsible for numerous deaths.
Uncle Tom's Cabin, "the book that started the Civil War," is a novel aimed at women in an attempt to get them to convince their voting husbands to outlaw slavery. Many times the narrator will address the reader directly to push her down this logical path.
William Lind's Victoria: A Novel of 4th Generation War is very much this, a story set 20 Minutes into the Future where the US dissolves into a couple of far-right and a number of straw liberal states, with the former proceeding to smash the latter to Kingdom Come.
Vita Brevis: A Letter to St Augustine by Jostein Gaarder consists of letters criticizing the works of an early Christian theologian, written from his fictional lover's point of view. His beliefs about sex and joy are contested in particular, and often in a way that might be seen as an appropriate reaction to mindsets still relevant, thus instrumentalizing 1500ish-year-old texts to point out present-day hypocrisy.
War and Peace was the means by which Leo Tolstoy wanted to share his view of history and historical forces. No no, the title doesn't give it away. What gives it away is the 100-page epilogue that drops any pretense of plot, characterization, drama, or interestingness. It even critiques the rest of the book directly.
The third book of The Witchlands suddenly has a lot of characters calling out patriarchy and the way some men manipulate history to put down women, after the previous two books and a novella have all but ignored the topic, save for a small subplot in book two.
A Wolf in the Soul is ultimately the author's treatise on what it means to be Jewish and what it means to be a human being.
Youth in Sexual Ecstasy is a novel dedicated to young people that heavily promotes sexual abstinence and a strong pro-life message.
The way in which events play out conform with the Elder Zosima's idea expressed throughout "everyone is guilty for all and before all."