The most relevant book to accompany T. H. White's The Once and Future King is, of course, the book on which it is based: Thomas Malory's Le Morte d'Arthur, although this older version of the Arthurian legend is written in rather old-fashioned prose. Yet Arthuriana is a subject that has inspired many writers. Students might want to check out J. R. R. Tolkien's The Fall of Arthur. Students with weaker reading skills might dip into the compendium of Arthurian tales in King Arthur: Tales from the Round Table (Dover Publications, 2002). Note, too, that White wrote a kind of sequel in The Book of Merlyn, which students might want to look at.
Those who are interested in the historical...