Sword of the Rightful King
Gawain is a Companion of the new king, Arthur, but he still falls under the rule of his mother, Queen Morgause, who is one of Arthur's many enemies and one of the few who are a threat to him. Merlinnus, advisor to the king, is fully aware of just how many oppose Arthur's rule, and so devises a plan. Like any wizard's plan it should have been perfect, but it is thwarted by Gawen, a mysterious young boy.
I found this book in the children's section of the library long before I ever dreamed of entering the young adult section, where I believe this book should be. I enjoyed it anyway and, years later, purchased my own copy at Borders. It is still good now that I am older.
There is something about this book that makes it impossible to classify as "too old" or "too young" for a certain group of people- it is simply a story everyone should know.