My daughter is an avid reader, and she is 12 years old. She has enjoyed the Series of Unfortunate Events, Little House on the Praire, Judy Blume and the American Girl Books.
She also loves the classics like Black Beauty, Ol' Yeller, Anne of Green Gables etc.
She also like books that are a little different from the norm, such as Diary of Anne Frank, the Outsiders, Animal Encyclopedias, Marley and Me and books about history (like fashion through the years, or women's rights, or the cival right movement)
And don't forget the Harry Potter series (always a winner!)
I would warn you (just a little) that since this is a gift for someone other than your own child, to stay away from some of the series that deal with life as a teen (preteen). I flipped through some of these while trying to see if my daughter would be interested, and was a little shocked to see some of the situations, and scenarios that these characters were in. I just wouldn't want to offend anyone.