Do I need to mark the book as #gifted?

You need to make it clear that the book has been sent to you for free, but not #gifted, as this is not an accurate representation of the transaction. To say something has been gifted to you, is to say that its a present, where as these books are sent to you in exchange for a review / socials post, therefore an advert. The best way to mark it as #ad, #prproduct or #adprproduct.