You are undecided. "I am not sure" is not "joyous yes!" sounding to me.
And you know your partner better than any internet stranger would!
In your shoes? I'd go with gift card, tell I enjoyed looking at the poly books, I was not sure she might want one of those titles, but I wanted her to enjoy bookstore time since I had fun at the store and was thinking of her. Then give it to her and let her pick what she likes.
If your want is for BOTH read the same book, that's another question. If that is the case? Seems easier to ask.
"I'd love to read a book together so we could discuss. Are you up for that? I wanted to read (name of book.) Could you be willing?"
And then see what she says.
This doesn't have to be hard.