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.
Anachronism Stew: Deliberately set in no particular time period, with historical references being often vague and frequently contradictory.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.
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.
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.
Bring News Back: Why Thomas Malory can not fight and die in the last battle as he tells Arthur he wants to.
Cerebus Syndrome / Darker and Edgier: The Sword and the Stone is pretty light-hearted, and not much different than the Disney film in terms of atmosphere. The story darkens from the second book onwards. White also intended the books to be poetic, comedic, dramatic and tragic in order.
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.
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.
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 Octahedron: It's not just Arthur/Guenever/Lancelot, it's explicitly Arthur/Guinever/Lancelot/God, with occasional complications from Elaine and Morgause.
Pre Ass Kicking One Liner: Lancelot has one in Candle in the Wind, before he goes out to fight the knights who are confronting him about his affair with Guinevere:
"Ah sirs," he said with a grimness, "is there none other grace with you? Then keep yourselves."
Public Domain Character: Apart from the obvious, Arthur meets Robin Hood (who says his name is really Robin Wood, but it's clearly the same person the legends are about) in The Sword in the Stone.
Reality Subtext: White wrote the majority of the series, including The Book of Merlyn, during the darkest days of World War II. It definitely shows, if you're looking for it. The scene with the ants in The Book of Merlyn is an explicit parallel to totalitarian regimes and Nazism in particular. And Mordred's faction in The Candle in the Wind are described in ways intended to compare them to Nazis. Plus there's Merlin's angry lecture to Sir Kay in The Queen of Air and Darkness, although that's less subtext than outright text.
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.
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.