I would say that if you ask once, casually, something like "have you started bookName yet?" or something, is not pushy (maybe wait a few weeks first though). I would feel you are being pushy if you ask me many times, specially if it was a gift and not a loan.

That being said... if you are gifting me a book, it will posible take me months or even years before starting it, I have a bunch of unread books on my shelves, and it wouldn't mean I am not interested in the book you gave me.