If you pay for a book on Google Play Books, your significant other should be able to read it, too. The same goes for movies, music, and even apps or games—if you make a purchase, everyone in the family should be able to enjoy it. Thanks to Google Family, they can.
Google Family lets families to share content across their Google Play accounts. You can have up to six accounts in your Family, with two primary types of accounts available within the Family. There are family managers—generally the parents—and then limited “Member” accounts, which are usually the children (or anyone else you don’t want to allow full access).