For anybody who liked Harry Potter, and for many who didn't, I warmly recommend the Tiffany Aching books by Terry Pratchett. The first one is The Wee Free Men.
The witches were always my favorite characters of Pratchett's diskworld books. Unlike, say, the wizards, they are tough and dead-sharp people who know what they are doing. But of course written about with Terry's usual warm wit and inventiveness. So imagine my delight when some years ago a new and fresh witch character came around, and one of the toughest and sharpest ones ever, in the unlikely little frame of Tiffany Aching, who was only nine in the first book. She grows older as the books go on though, in the latest, I Shall Wear Midnight, she is sixteen.
Pratchett said that the books are "ostensibly" for young readers, and I couldn't agree more. There is nothing to distinguish his "young reader" books from his others, except that the characters are young and I imagine a young reader would love to grow up on them, like kids did with Harry Potter. But even experienced readers may find words and concepts he'll have to digest a bit, there are no weak blows here.