Nobody's mentioned T. A. Barron's The Lost Years of Merlin series yet? It's not quite Arthurian, but tracks a young Merlin's development, the development of his second sight and how he comes to grips with powers he doesn't want. The first one was quite well done. (One of those I keep meaning to get back to...)

Jane Yolen also wrote a trilogy on Merlin's origins, the Young Merlin trilogy.