T.H. White did an excellent job of capturing the feeling of England during the time of King Arthur and, with a little willing suspension of disbelief, the book could be considered historical fiction. It makes me feel like I'm actually meeting the characters, real and fictional, and watching the exciting events in their everyday lives.
The book has quite a bit of romance and some violence, so it's not appropriate for younger readers but lots of kids old enough to read over 600 pages will enjoy it.
It's easy to see why this version of the classical Knights of the Round Table stories has become a classic itself.