I'd suggest not giving her a book overtly about atheism. Something more broadly about skepticism would be a better choice I think. The Demon-Haunted World gets a lot of recommendations, though I haven't actually read it myself.
In non-fiction, The Philosophy Gym might be suitable. I've read the first three chapters and they seemed fairly clear-thinking. (I was reading with an eye to writing a review; it's been a month or so since I picked it up, but there was no particular reason I stopped.)
If it's worth saying, but not worth its own post, even in Discussion, it goes here.