For those of us interested in book news from across the pond (that is, the UK), there's an online monthly magazine--the Gateway Monthly-- that is chock full of information, reviews, contests, etc. The March issue has just been put up. I was especially pleased to see that The Sword in the Stone , by TH White, is being re-published with a jazzy new cover; I love that book and hope it finds new fans.
The Sword in the Stone tells of the boyhood of King Arthur, and the story continues in The Once and Future King. However, a version of The S. in the S. can be found at the beginning of The O. and F. K., but the two are very different, and the S in the S is much, much better and more child friendly.
The S. in the S. version was T.H. White's final choice. The Book of Merlin, published after these two and continuing Arthur's story, uses material that appears verbatim in the The O. and F. K. version (Arthur's journeys to the ants and the geese, which are so much more effective when they happen in his old age. They also make much more sense to adult readers than to children).
So, to summarize: read The Sword in the Stone when you are about 9 or 10, then The Once and Future King when you are about 14 or 15, skipping the part about Arthur's childhood, then read, when you are even older, The Book of Merlin.