The Once and Future King is a retelling by T. H. White of the story of King Arthur. It is considered one of the best retellings of Arthurian legend. It was originally published as separate books from 1938 to 1941 and collected in one volume in 1958. The 1958 version contains:
The Sword in the Stone, covering Arthur's childhood, the lessons he was taught by Merlyn (in which he transforms him into animals in order to give him a different perspective on the world), how he was discovered and crowned King of England.
The Queen of Air and Darkness, covers the early part of Arthur's reign, the founding of the Knights of the Round Table, and introduces Morgause, the mother of Arthur's nemesis Mordred.
The Ill-Made Knight, concerning Sir Lancelot.
The Candle in the Wind, telling of the downfall of Arthur and his kingdom, concluding with a bit appearance by Thomas Malory, still a squire, whom Arthur sends off to remember their story.
The first three parts were published separately first, and revised to a greater or lesser extent for the collected edition. The biggest change was probably to the second part, which was substantially altered and given a new title. (The original version had been entitled The Witch in the Wood).White also worked on a fifth part set before Arthur's final battle, in which he was taught more lessons by Merlyn. Parts of it were incorporated into the collected edition's version of The Sword in the Stone. In 1977, after White's death, it was published separately as The Book of Merlyn.The Sword in the Stone was loosely adapted into a Disney film of the same name. The musical Camelot is based on The Ill-Made Knight and The Candle in the Wind.
The Once and Future King provides examples of the following tropes:
The Ace: Deconstructed with Sir Galahad, who is so inhumanly perfect that he is disliked by most of his fellow knights.
Adaptation Dye-Job: Guinever is stated to have jet-black hair, although the narrator admits that in most stories, including the author's source material, she is blond.
Anachronism Stew: Deliberately set in no particular time period, with historical references being often vague and frequently contradictory. In several cases, White justifies it by saying that some things referenced (such as the characters drinking Port or wanting to send their kids to Eton) weren't actually what was being said, but that more modern things were used to give readers a sense of what was being said. Of course, this version's Merlyn lives backwards through time, so he possesses objects and knowledge of the future (a certain madman who caused WWII, for instance, and he keeps guns in his house, as well as dressing in clothes that are unknown to those around him)
Ant War: One of the animal transformations Merlyn performs on Wart is to turn him into an ant, and he finds himself in the middle of an ant war. Luckily he gets out of it before things turn nasty.
There was a real corkindrill hanging from the rafters, very life-like and horrible with glass eyes and scaly tail stretched out behind it. When its master came into the room it winked one eye in salutation, although it was stuffed.
Argument of Contradictions: Sir Grummore and King Pellinore have a rather lengthy one following a battle, first about whether Pellinore said "Pax" or "Pax Non," then about whether or not he's a cad, then a bit later about him being a cheat. The whole thing continues on and off for about a page.
Badass Creed: The song of the hawks in The Sword in the Stone.
Bi the Way / If It's You, It's Okay: Lancelot was in love with Arthur, though one might argue what "in love" specifically means. His jealousy and hostility towards Guinevere actually plays a part in them ending up together.
Book Ends: The 4th book ends with Arthur reminiscing about his lessons with Merlyn as Wart- specifically, the difference between the pacifist geese he flew with, and the militaristic ant colony.
Bring News Back: Why Thomas Malory can not fight and die in the last battle as he tells Arthur he wants to.
Classical Mythology: Several personages from Greco-Roman mythology, such as Neptune, Hecate, and Minerva, make appearances in The Sword in the Stone (not to mention Castor and Pollux blowing Merlyn to Bermuda).
Deus Angst Machina: Everything that happens to Lancelot and much of what happens to Guenevere. In the concluding book Arthur, Guenevere and the Orkney brothers (excepting Mordred) all go through hell.
The Fair Folk: Before Gaiman did this, the fairies in The Sword in the Stone were one of the earliest examples of these in modern fiction. Robin Wood said that they didn't have hearts, both literally and figuratively.
Famed in Story: Arthur wanted the story told, at the end, to keep the memory of the ideals alive.
Forgotten Fallen Friend: In the fourth book, Lancelot kills Gawaine's brothers Gareth and Gaheris. Though Gareth's death is devastating to both Gawaine and Lancelot, neither of them seems to remember that Gaheris ever existed. (Or any of the brother's wives, who reportedly exist, but ever appear in the book.) Sir Kay's death goes pretty much unnoticed as well. (Unless White means a different Sir Kay.)
For the Evulz: the "motivation" of Mordred, Morgaine, Morgause (all related) and many walk-on villains.
Gorn: Queen Morgause boils a cat alive out of boredom. She doesn't even care about the practical purpose of granting invisibility, as she's intensely vain.
This is also a highly effective Kicking of the Dog, since the second book (and her first scene) opens with her doing this.
Individuality Is Illegal: The ants Arthur meets in one of Merlyn's lessons. "EVERYTHING NOT FORBIDDEN IS COMPULSORY." White flat-out states that the ants are Communists.
Jerk Ass: Sir Kay, even as a little boy. It's somewhat understandable, seeing as he grew up under a wealthy lord and was spoiled.
Just So Story: The badger's dissertation in The Sword in the Stone.
Kavorka Man: Sir Lancelot. He is known as Le Chevalier Mal Fet, "the Ill-Made Knight", as he is short, ugly and quite possibly ridiculously bow-legged. Yet he is loved by Guenever and Elaine, and admired by many other ladies as well.
Lampshade Hanging: Merlyn does this constantly, since he's basically Genre Savvy due to moving backwards through time. Some of his most marvelous ones spiral off into funny and inspiring sermons about learning and the nature of spoken language.
Level Ate: In The Sword in the Stone, Arthur and Kay enter a faerie castle made of food.
Nausea Fuel: Said food turns out to be lakes of butter, walls of cheese, and the Fairy queen is found on a bed of lard.
Narrator: It was horrible beyond belief-sweet, sickly, and pungent- so that they did not feel the least wish to swallow a particle of it. The real temptation was to run away.
Love Triangle: Arthur/Guenever/Lancelot, of course, although it doesn't fit any of the traditional Triang Relations as it's made very clear several times that while Gwen and Lance are very much into one another, they both love Arthur too (and he explicitly says that he loves them both dearly as well).
Love Dodecahedron: It's not just Arthur/Guenever/Lancelot, it's explicitly Arthur/Guinever/Lancelot/God, with occasional complications from Elaine and Morgause.
Motifs: several, one early one being the crow with an arrow in its beak, which symbolizes trouble to come. It's first seen catching the Wart's arrow, upon which Kay claims it was a witch's doing. The same crow is seen at Morgan le Fay's castle, and then a crow-with-arrow weathervane sits atop the Orkney castle.
Sacrificed Basic Skill for Awesome Training: Lancelot devoted most of his youth to learning how to fight, at the expense of having a proper childhood. He's better with a weapon than anyone else in the book, but among other things, he never learned to climb trees.
Spell My Name with an S: Most notably "Merlyn" (with one discrepancy in the poem before Book 1, where the name is "Merlin") and "Guenever".
Translation Convention: Explicitly invoked by the narrator. In the first few pages the narration states that characters are actually using or referring to certain period-accurate things like some kind of drink, but the narration will translate it into a modern equivalent, like port. In addition, most of the dialogue is in modern colloquial English, but for a few important parts here and there it switches to something much more old-fashioned-sounding. There is no in-universe explanation for this; the switch is probably just to drive home the point "this is important".
Virgin Power: Invoked by Lancelot, believing this is the case early on in the Ill-Made Knight.