The Gone books. Each book seems to get progressively darker, except for Lies, which was about on the same level as Hunger. Not that it started out on a light note, though. Fear is this on a very literal level.
The California Diaries series, compared to The Baby-Sitters Club. However, the use of this trope surprisingly didn't come off as cheesy or overdone. It allowed for more character development and exploration of realistic adolescent themes, like depression, drifting away from childhood friends, and (arguably) closeted homosexuality.
The Berenstain Bears books normally come in the form of small short books that deal with small family issues like being afraid of the dark at night and way too much junk food. But they also had mini-chapter books that dealt with slightly darker themes like shoplifting, friendships going sour, political controversy, and the destruction of natural habitats.
Diana Gabaldon's Lord John Grey series, historical mysteries concerning a secondary character from her main set of historicals, come across as an attempt to be both Darker and Edgier and Hotter and Sexier, using the seedy aspects of the protagonist's forbidden love affairs, him being gay and the setting being the 18th century, for all the shock they're worth. They may or may not have managed it. (Her main books are themselves essentially Darker and Edgier versions of the 'roguish Scots in kilts' type of historical romance, though significantly better written- there's still smoldering glances, kilts, time travels and duels, but the male love interest's the one who suffers all the traumatic villain-initiated rape scenes and Gabaldon doesn't hold back on the gore or inequality much.)
Wicked and The Wonderful Wizard of Oz. The Wonderful Wizard of Oz is a fluffy, heartwarming story of a few friends in a magical country. Wicked, the novel, doesn't go more than a few pages without some swear word or mention of sex, or just sex. Gregory Maguire had a pretty dirty mind... there is a lot of weird romance in it, like Elphaba's father and mother were both in love with the same man, Elphaba's roommate was in love with her (but married a older rich guy, who all Gelphie shippers insist is an abusive ass), Elphaba's guy friend and his friend may have had a hint of romance... it never ends.
And yet this isn't the actual thrust of the plot. The Wizard is a tyrant, using a secret police and assassination to suppress dissension and many ethnic groups. Conscious, sapient Animals are sent to farms and stripped of their rights, resulting in many Animals going into hiding. Elphaba herself is willing to commit murder to help her cause, and works for what can only be called a terrorist group at one point. Her mentor, Doctor Dillamond, is brutally murdered for coming close to proving the minor point that Animals (the sapient kind) and animals (the normal kind) and humans are made from the same stuff. Religious tensions between Tick-tokism (straw-man science), Lurline (straw man paganism), and the Unionists worshiping the Unnamed God tears apart society. The Wizard's projects come at severe cost in life, such as the destruction of the Quadlings' country for ruby mines. Racism between humans - especially towards Winkies and Quadlings, is common (though Munchkinlanders of means always "marry into height)." The land is caught in a terrible drought. The Yellow Brick Road and Emerald City are both wasteful boondoggles. Witch sex is hardly the 'darker and edgier' in Wicked.
And the original Wizard Of Oz book isn't as "fluffy and heartwarming" as many might think from seeing the 1939 musical film. In the book, for instance, the Tin Woodsman is made of tin because when he was a normal human, the witch enchanted his axe to repeatedly cut off various parts of his body which he kept replacing with tin. Also, the witch enslaves Dorothy and her friends at one point.
Another Gregory Maguire novel, Mirror Mirror, about Snow White has lots of kink. (Menstruation does not work that way!)
Neil Gaiman gave Snow White a similar treatment in his short story "Snow, Glass, Apples."
The Harry Potter books tended to get Darker And Edgier as they went along. Which was no accident. Rowling set out to write a series that would grow up with its audience, and it was published over a decade — so the same 10-year-olds expected to read Philosopher's Stone were expected to be about 20 when they read Deathly Hallows, and ready for more mature fare. Naturally, this was entirely lost on most Concerned Parents, leading to oodles of Fan Dumb and What Do You Mean, It's for Kids?. This started with a noticeable difference between The Goblet of Fire and The Prisoner of Azkaban.
Many of the poems in Songs of Experience are darker counterparts to poems in Songs of Innocence, for example "THE Chimney Sweeper" to "The Chimney Sweeper", "Infant Sorrow" to "Infant Joy", and both "The Human Abstract" and the cut poem "A DIVINE IMAGE" to "The Divine Image".
After the success of her second novel Pride and Prejudice, Jane Austen wrote to her sister Cassandra that she felt it was "too light, and bright, and sparkling" and planned to write something different next time. The result was her most realistic and controversial novel, Mansfield Park.
They were even Darker before the Grimm brothers got a hold of them too.
The Dresden Files, while never sparkles, rainbows, and kittens, is getting darker. Genocide being the most recent inclusion...
And that's the protagonist's doing. The villains have gotten considerably larger-scale as well, but the constant character development justifies all of this. The turning point seems to be post-Grave Peril (book three), then changes again post-Dead Beat (book 7), and seems to be headed that way post-Changes (book twelve).
The Dresden Files, already far from a fluffy saccharine series, got a fair bit darker with Ghost Story. Murphy's withdrawn and hostile, Molly is a few fries short of a happy meal, and the Fomor are a lot like the Red Court without the love and sense of fair play.
At one point, Harry implicitly uses this trope when he needs to get himself past a truly horrifying thing he saw with his Sight. He reminds himself of all the darker and edgier (and painfully beautiful) things he's seen, and eventually can deal with this as just the next in a long line of darker and edgier increments. For those who haven't read the books, anything seen with a Wizard's Sight is unforgettable - they can never see it with any less clarity for the rest of their very long lives.
Sometimes, oddly enough, justified in universe. Harry's behavior for a few books was due to him being possessed by a Fallen Angel. Molly's demeanor in Ghost Stories is actually in some part an act that she's using to try and be as scary to bad guys as Harry was.
Harry occasionally mentions this, either to the readers or to Ebenezer McCoy. It's part of how he reasons out that the Black Council has to exist.
As of the latest novel, Cold Days, the tone has lightened slightly from the previous novel, although it is still very dark and edgy.
Jim Butcher's other series Codex Alera does this as the series progresses. In the first novel, we have Tavi undergoing a personal challenge. By the end, there is a empire-wide war going on against the Vord where the very existence of the empire is threatened.
Weirdly enough, the Cambridge Latin Course textbooks. The first book were mostly just fairly light-hearted stories about Caecilius and his family, all of whom come across as genuinely nice people... until the last chapter, when Vesuvius erupts, killing almost the entire cast (even the dog!) The next book moves to Roman Britain, where a new main character, Salvius, is introduced. In his very first story, he executes one of his slaves for the heinous crime of being too sick to work, and things mostly go downhill from there. The final book ends with Salvius being taken to court for fraud and attempting to commit suicide to keep his honour intact.
Caecilius was a real person, and he did probably die in either the eruption of Vesuvius or an earlier earthquake. They had to stay true to history if they wanted to use someone who had actually lived...
The Star Shards Chronicles trilogy starts out with some fairly dark horror themes, but stays PG-13. The final book, however, turns up the sex-and-profanity dial quite a bit.
A series of original novels based upon the Tomb Raider games was published in the mid-2000s. While the games themselves had become darker and edgier over time, the novels fully recast Lara as a killer more than an explorer and archeologist. One novel, The Man of Bronze, is particularly violent, with Lara describing in first person how she mercilessly kills a group of thugs (in the process recalling how she once killed a man while kissing him). Later, she attempts to kill a man in cold blood for apparently no other reason than he was painting a sexy portrait of her (she is unsuccessful).
Stuck starts off fair enough, though in its final episode the themes get darker and there's a bit more violence and black humor. Not surprising, considering that the main characters become fugitives.
The Cinderella adaptation Sunny Ella casts Cinderella as a deluded murderer and Rapunzel as a soulless half-vampire.
The Nancy Drew Files and The Hardy Boys Casefiles spin-offs weren't really an attempt to go Darker and Edgier, but a switch to a new publisher removed most of their previous roadblocks, namely Never Say "Die", No Hugging, No Kissing, and the like (however, they were an attempt to skew older, hoping that maybe young readers might graduate up to them after aging out of the original series intended demographic.) In doing so, they also got better written as a side effect, and fans of both series consider them some of the better books in their respective franchises.
Darke of the Septimus Heap series is noticeably darker than the preceding books, what with the existence of the Castle being on play and lots of people dying in the end.
Being a Warhammer40k series, Gaunt's Ghosts was never sunshine and rainbows, but starting with The Guns of Tanith things got noticeably more brutal and grim, with beloved characters dying off, the battles getting even more desperate. Compare series starter First & Only with book 8, Traitor General, and you could almost be forgiven for thinking you were reading two entirely different series.
The Lorax is this compared to the other Dr. Seuss books. It teaches about the consequences of not acknowledging natural resources until they are gone.
Even more so, The Butter Battle Book is a parable about the nuclear arms race between the US and the USSR, represented by two tribes living on opposite sides of a wall analogous to the Berlin Wall, concluding with a Bolivian Army Ending as the two sides are about to drop their bombs.
Cahills vs. Vespers, the second series of The 39 Clues, takes some noticeable liberties with language, violence, and romantic relationships as compared to its preceding series.