There are three steps:
First, find what genre she likes the most and which books are her favorites. It’s a fun conversation to have.
Second, find out which books are more similar to her favorites and what other books are liked by people with the same taste as them. You can ask someone who is a big fan of the genre or find similar books in “recommendation” section sites like GoodReads or Amazon for each book that she likes.
Third, casually ask her if she had heard about the books you have chosen. make sure that she didn't read the book which you are going to buy.
Now you know which one she might like the most and can be a great gift.