“The Merry Adventures of Robin Hood” by Howard Pyle
“The Sword in the Stone” by T.H. White (this book covers the story only up until he becomes king – I am anxious to see what books people recommend for the king/knights of the round table years)
Both books are advanced-style literature, but my son still understood and enjoyed them being read to him around age 10. Unless the 12-year-old is an excellent reader, they may be better as read-alouds or audio books, but they are THE BEST in my opinion!!