Wow! I am, essentially, seconding everything here. ;) Tamora Pierce was what turned me from a reader into a Serious Reader -- not the kind of reader who reads serious books, but the kind of child reader who would occasionally skip recess to read -- and Diane Duane's books probably changed my life. Patricia C. Wrede's stuff is so good that I like re-reading it even now, and it's really funny besides; Diana Wynne Jones does wonderfully wacky stuff that no one else can pull off.

As for boys -- not that girls won't love these also, because obviously I did, but I've never met a boy who liked Tamora Pierce much unfortunately -- T.A. Barron's Merlin books are excellent. Also, Orson Scott Card. <3

Banui, you are the Good Samaritan of all things literary, you know that? ;) Yay for improving young reading minds!