On the science fiction side, I would recommend looking at Connie Willis, CJ Cherryh's Cyteen series (three books, read them in order), Lois McMaster Bujold (in particular the Miles Vorkosigan series), and the Robert Heinlein juveniles (the adult-oriented ones are pretty racy, but he did write a number of books for younger readers).
Definitely the Madeleine L'Engle books recommended earlier! Also Scott O'Dell wrote quite a few in that line.
If she would be interested in any fantasy, Piers Anthony has a number of fantastic series. The Apprentice Adept series (7 books) is terrific, as is the Incarnations of Immortality (also 7 books). There is a very interesting and completely different type of series called Geodyssey (I think it's 5 books now) that I find fascinating and impossible to explain. And of course, for the punny at heart, there is always his Xanth series (too many to count). Along those lines, Robert Asprin has a punny light fantasy series called the Myth books--Mythconceptions, Mything Link, etc.
Hope that helps!
