The app she has is called Bookworm (not to be confused with the game of the same name) and I'm pretty sure it was free (she's with Verizon). I had a chance to play with this app over the weekend.
You can manually enter book info or, and my favorite, you can scan the barcode! Then the book will be added to the database. You can sort the database by author or title (and other things I think but I can't remember). You can mark the book as read and I believe you can rate it also.
I think this app would be a ton of fun! If you see a book you are interested in at the bookstore but don't plan on buying it, just scan the barcode and save it for later. Read a lot of series and sometimes get confused on which books in the series you have read, just add it and next time you get ready to buy a book in that series, you'll know which ones you have already read.
If I didn't already want a smart phone, this would really make me want one (and the game Angry Birds..don't get me started on that game...talk about addicting).