I thought I'd mention T.A. Barron's Merlin book series, which explores what might have happened to Merlin during his youthful years, before he appears fully grown and fairly wise in our more traditional Arthurian legend. The first book, The Lost Years of Merlin, is my favorite, although I hesitate to say it's the best in the series -- it holds the most nostalgia for me, as I read it when I was young and before the rest of the series was published, then went back years later to read them all.
The rest in the series:
The series is pretty fun, although clearly aimed at grade school audiences. (Which is good for grade schoolers but less in-depth for people my age.) Barron does a pretty good job of filling in a mythological blank without treading on the established legends (for what they're worth, as they tend to change pretty easily). And it looks like since I read the series, Barron has returned to the world with another character in an in-between type adventure: Merlin's Dragon.