It depends on what she is looking for in a guide book.
Growing up, we always got the Birnbaum guide. Lots of cute pictures, very basic info. But it is fun toook at the bookshel and see 1988, 1994, 1997,2002...and then the internet was invented.
If she has already read the UG, I suggest the UG Color Companion. It is like the UG, but with the pictures saying a thousand words each!!
If she is looking for something more informative than planning in nature, I loved the Imagineers Field Guide set. There is one for each park, and they are pocketsize. They really go into detail about how and why things look the way they do and lots of things that might be missed! The only downside to those is because of the size, the pictures and some of the text are small. But I understand, they wanted something you could carry in the park without being to cumbersome.