King Richard the Lionheart survived the crossbow wound and subsequent infection that killed him in our history, but the narrow escape caused him to reconsider his life and become a famously great monarch. Nobody ever got around to agitating for Magna Carta, and in the twentieth century the Plantagenet dynasty still rules absolutely in England — not to mention France, most of the rest of western Europe, and North and South America.
The course of scientific discovery went down a different path, with the result that magic and psychic powers are well-understood phenomena with clearly-defined rules (but nobody knows much, or cares, about the physical sciences).
As a result, in the 1970s, the Angevin Empire's society and technology largely resembles those of, to pick a comparison entirely at random, the Sherlock Holmes stories. But with wizards.Lord Darcy is an official investigator for His Highness the Duke of Normandy, solving mysteries too weird or too politically sensitive for the normal police to handle. He is ably assisted by Master Sorcerer Sean O Lochlainn, a one-man magical CSI department.In the later stories, Lord Darcy and Master Sean increasingly often become entangled in the espionage and counter-espionage of their world's version of the Cold War, between the Angevin Empire and the ambitious-but-not-strong-enough-to-wage-conventional-war Polish Empire.Lord Darcy was introduced in 1964. Murder and Magic (1979) and Lord Darcy Investigates (1981) collect all of the Lord Darcy stories, bar two that were published later. Garrett also wrote one Lord Darcy novel, Too Many Magicians (1966), which fits between. A 2002 omnibus edition collects all three books and the two stray stories.In the 1980s, following Garrett's death, his friend and fellow-author Michael Kurland wrote two more Lord Darcy novels, Ten Little Wizards (1988) and A Study in Sorcery (1989).
The series provides examples of:
Allohistorical Allusion: In one of Kurland's novels, a character views paintings of Plantagenet monarchs past, and speculates about how horribly history could've gone, had King Richard died sooner and the throne had passed to his Jerk Ass brother instead of his nephew.
Always Murder: Well, mostly. Subverted in "The Sixteen Keys", where it turns out they already know what the victim died of, and just need Darcy to figure out where he'd hidden an important document before he snuffed it.
Asshole Victim: Count D'Evreux, the victim in "The Eyes Have It", has many personality flaws. The underling who finds his body says outright that he'd always expected the Count to wind up being done in by somebody.
The Baroness: Olga Polovski, Agent 055 of the Polish Secret Service.
Black Magic: Combined with Saintly Church, in that performing any kind of harmful deed through magic (even injuring a criminal in defensive combat) causes irreparable mental damage of some sort. The clergy function as magical psychologists of a sort; though they can't "cure" people of the resulting (or causative!) evilness, they can magically detect psychosis and render such people incapable of working magic to prevent further harm to a black magician's mind. As a result, practicing magic requires an official license gained through examination by the clergy.
Blue Blood: Extremely prevalent, and even explained in-universe—the attentions of investigators such as Lord Darcy are required for crimes involving members of the aristocracy, while Armsmen (essentially, local cops) handle the investigation of crimes among the common people.
Clarke's Third Law: Inverted. There are instances of devices that work on clearly understood principles, in our world, but in the Darcy world, their sages have no idea how they work, just that they do. Examples include the teleson (a telephone), and a device created by a top secret military research program: a flashlight.
Master Sean is also accused of the murder in Too Many Magicians, but it's only a ploy by the Marquis of London to get Lord Darcy to come and investigate the murder for him.
Combat Clairvoyance: Commander Lord Ashley, in Too Many Magicians, has just a touch of magical talent that gives him the occasional burst of prescience. The most dramatic presentation of this power comes when he's fighting an opponent armed with an enchanted sword that's effectively invisible; Ashley's power lets him accurately predict the other man's movements.
Dead Man's Chest: The woodworkers preparing to deliver the coffin of the Duke of Kent in "The Muddle of the Woad" are surprised to find that a body's already been stashed in it.
Detective Mole: In Too Many Magicians, the murderer is one of the people conducting the investigation; the people he murdered had each discovered, or were about to discover, that he had been subverted by the Polish secret service.
For Want of a Nail: Richard the Lionheart's survival resulted in the discovery that led to the harnessing of magic. (It might be easier to imagine a timeline in which the harnessing of magic resulted in Richard's survival, but it's explicitly stated that in this timeline Richard's survival came first.) The explanation is that Richard's brush with death caused him to change his rulership style. For the remainder of his reign, he encouraged learning and the arts. And this trend continued under his successor; his nephew Arthur. It was the academic environment fostered by Richard and Arthur that led to the discovery of the rules of magic.
Funetik Aksent: Polish characters who aren't trying to disguise themselves as Angevin subjects tend to have this.
Gentleman Wizard: Several minor characters (including a couple of victims). Recurring character Lord John Quetzal is an interesting case, as he's a nobleman and a gentleman, but he's from the colonies (Mexico, in our version of reality), which gives him some interesting quirks.
Locked Room Mystery: At least half the series. Notably, in spite of the obvious temptation, the answer is never "A Wizard Did It" (Though in one case a wizard tried to do it, but failed because someone else did it first).
Magic A Is Magic A: Magic is so systematized that performing a particular spell will always have the same result.
In one story, Master Sean enchanted a splinter left behind by a murder weapon and used it to find the rest of the weapon.
Too Many Magicians has Master Sean give Lord Darcy a "tracker", a piece of wood broken and enchanted so that the holder of one piece can tell where the other is.
Another character has this as one of his magical abilities—once he locks onto someone, he can tell which direction and roughly how far (up to a certain range) the target is from his location.
Majored in Western Hypocrisy: Several instances, though all without the hypocrisy part because the Angevin Empire is better than its counterparts in our world. The most prominent example is Lord John Quetzal, a native American nobleman who is studying in London in Too Many Magicians. The "character wrong-footed by foreigner's education" version appears in Michael Kurland's A Study in Sorcery, where Lord Darcy meets a woman who has invented a dramatic past for herself that includes a stint in the harem of the son of the Osmanli Sultan; in the course of dissecting her story, he mentions that he and the son of the Osmanli Sultan were at Oxford together.
Miming the Cues: In "The Napoli Express", a murder is committed while Lord Darcy is undercover, leaving him unable to take official charge of the investigation. While another detective leads the investigation with Master Sean's assistance, Lord Darcy uses hand-signals to prompt Master Sean to suggest "what Lord Darcy would do next if he were here".
Modern Mayincatec Empire: Garrett left the state of things in the Americas largely undefined, but Too Many Magicians mentions that the Aztec emperor Montezuma's descendants now rule "Mechicoe" as noblemen of the Angevin Empire. Michael Kurland's A Study in Sorcery, being set largely in North America, is much more specific, and adds that part of the Aztec Empire continues unabated farther south.
Noodle Incident: In "Too Many Magicians", a few of Darcy's unseen cases are mentioned in passing, but nothing more is revealed about them. Similarly, at least one case mentioned by a Fan Boy in "Ten Little Wizards" is not covered in the extant stories.
Origins Episode: "The Spell of War", one of the last-published stories, recounts the first meeting of Lord Darcy and Master Sean on a battlefield during their world's equivalent of World War II.
Only One Name: Lord Darcy's given name is never revealed. In "The Spell of War", where he's not yet inherited his title, he's referred to as "Lieutenant Darcy".
Orient Express: "Murder on the Napoli Express" is set on the Angevin Empire's counterpart to the famous train.
Royal "We": King John IV uses this when speaking as King-Emperor and drops it on those rare occasions where he needs to speak man to man.
Saintly Church: Apparently magic makes it possible to ensure that only suitable people become priests (and has presumably cleared up the whole is-there-a-God question, although that point is never really addressed).
Shout Out Literature: Various stories guest-star alternate universe versions of other famous detectives and secret agents, not to mention one or two other SF writers.
One example is found in the novel Too Many Magicians. My Lord the Marquis of London is an enormously fat man who grows rare flowers and leaves all the footwork to his assistant Lord Bontriomphe ("Goodwin", as in Archie Goodwin).
Smoking Gun Control: A character once comments that detective work in cities would be a lot easier without all the anti-scrying spells placed on homes and businesses. Darcy comments that if these were not there, detective work would be non-existent—you could just call in a journeyman sorcerer to use some basic divination spells and the case would be solved in under an hour. He also mentions that this would also eliminate all hope of personal privacy, as any interested mage could scry into your house or office whenever they wanted (this being the reason that anti-scrying spells are placed on homes and businesses in the first place).
Spell Construction: Although magic is limited to those with the Talent, actually casting a spell requires intricate and specific ingredients and actions.
The Stars Are Going Out: In "The Ipswich Phial", a top-secret magical effect makes Father Lyon think this has happened, by causing a sort of hysterical blindness in a localized area.
Stealth Pun: The infamous subplot in Too Many Magicians involving the uncle from the Isle of Man.
Summation Gathering: Most notably in Too Many Magicians, but also in several of the other stories.
Unfriendly Fire: In "The Spell of War", Darcy, a young officer at the time, chooses not to notice that the commander of his unit—who'd been a tyrant and endangered the men—had a low-angle bullet entry wound from a pistol... received while under fire from a sniper who was using a rifle from a high angle. (The soldier who killed him died almost immediately afterward, while the one piece of evidence—the body—is destroyed in an artillery barrage; Darcy chooses to protect the soldier's reputation.)
Witch Species: In order to practice magic, one must be born with "the Talent". This exists to varying degrees, such that only a small portion of the population can work magic, some others exhibit strange powers, nearly all can at least perceive strong magic to some extent, and a few on the other end are magically inert and utterly unable to directly sense the supernatural. Interestingly, the world's foremost magical theorist and expert in the symbolic manipulations underlying modern magic happens to be unable to work magic at all.
World of Pun: Not at the pun-per-paragraph extreme, but the series definitely keeps the punometer ticking away.
Especially when it comes to the literary shout-outs, which are often veiled behind the French equivalent of Canis Latinicus, as with master-spy James le Liennote "lien" = "bond", or the sedentary Marquis of London's invaluable assistant, Lord Bontriomphenote "bon triomphe" = "good win".
Also, one story has the local policeman suggest that the victim was attacked by a demon or fire elemental; Master Sean soon disproves the hypothesis, and it seems to have been thrown in largely as an excuse to have somebody say "Elemental, my dear Doctor".