One more series: the Redwall series by Brian Jacques, I'd start with either Redwall or Mossimo

What about the Madeleine L'Engle books (Wrinkle in Time, Wind in the Door, Swiftly Tilting Planet, and Many Waters)? I loved them when I was that age. Of course, that was a looooooooooong time ago.

I second the recommendations of the Ender's Game series, Xanth series (although it got racy at times), and Artemis Fowl books. They are all great series.