Most book retailers have books listed by age on their websites. We use Border’s, but I’m sure most have access. If she reads fast, you may give a longer lasting gift with a series of paperbacks rather than one new release hard cover – unless she has already read everything in the series and the latest version just came out – then there is a nice ‘wow’ factor with the hard cover!
I see Harry Potter, Inkheart, and Narnia series have already been suggested. Those were big hits here as well, and if she likes fantasy/adventure books as my son did at that age she may also like:
The Artemis Foul series (by Eoin Colfer - Time Paradox, Lost Colony, etc), Charlie Bone books (author Jenny Nimmo), and author Cornelia Funk (Dragon Rider is one I recall, but there are many). Author Rick Riordon (Lightning Thief, The Last Olympian, etc). Sorry, I’ve only been through that age with a boy, so there are probably more girl appealing books out there. It really depends on her maturity level and interests.
Another source of reading suggestions is "Battle of the Books", a program used in our school district as well as many others. Students ‘compete’ in teams with their knowledge of the books in the spring. A purchasing challenge may be that several of these books are not new releases and are harder to find, though our local Borders had a few on display for summer reading when I was in there last week. The good news is they are paperback, so less expensive. Here are links to their suggested reading lists for this year for grades 4-6 and 6-8.
P.S. If her school has a Book Fair during the school year, a gift certificate to it would be a fun gift to think about for future giving. They are usually held as fund raisers for the school so you would be giving twice =); the kids love to be able to pick out their own books and the school (or PTA) gets a percentage of sales to buy books for the school. Your 6 1/2 year old would probably find a book or two also!