I have read and re-read Terry Pratchett/Neil Gaiman’s Good Omens over the last 20 or so years. Simple but beautifully written book (in my opinion) that still makes me laugh and feel generally good about life. I was gutted when I lost my original copy and have replaced it accordingly.
I’m very fortunate in having a hardcover signed by both authors. Terry signed it first, and wrote; “Neil made me do it!” 😀
Try to find out, subtly, what her absolute favourite book is, then see if you can get hold of a good condition hardcover copy. Don’t try to find a book that you think she ought to like, get an original copy, possibly even signed, of something she loves and treasures.
A friend mentioned a couple of books that shed had as a child, but lost, and I managed to track one down in hardcover for her birthday this time last year. She was ecstatic at having it back.
I’ve got several books that I’ve read over and over again as paperbacks, but a mate has tracked down original hardcovers, and I really treasure those. Abe Books are a really good place to start, or just google titles and see what you get.