I'm not sure if you're one of those people who uses "the other day" loosely lol, but I would definitely wait a few months before asking. Then just casually ask if she's had a chance to read it yet.

My to be read pile is a mile long so if someone gifts me a book it could take me a couple of years to get to it. Doesn't mean I'm not interested, it just means I haven't gotten to it yet. So I would just bring it up once and if she hadnt read it at that point, then just drop it and wait for her to bring it up again.