But that's going to change soon.
Actualitte has uncovered an update to the Google Play Books ToS which mentions a "Family Library" sharing option. At the discretion of the publisher, readers will be able to share a book purchased in GPB with up to five other people.
2.1 The definitions of "Authorized End Users" or "End Users" (as applicable) are amended to include all members of a Family Group for the purpose of Family Sharing. As a result, the rights, authorizations and licenses granted by Publisher under the Agreement to Authorized End Users or End Users (as applicable) are granted by Publisher to every member of the applicable Family Group.
2.2 The rights, authorizations and licenses granted by Publisher under the Agreement to Google are amended to allow Google to make the applicable electronic book(s) available to every member of the applicable Family Group in accordance with the rights, authorizations and licenses that are amended under section 2.1 of this Amendment.
The new ToS also mentions that the six accounts will share a single payment method which will be under the control of a Family Manager, the user "responsible for managing various aspects of the Family Group account" including inviting new members of the group, approving in-app purchases, and kicking members out.
I haven't found a sign of the new sharing option in Google Play Books, but there are mentions of the new family group features in this week's update to Google Play Service (8.4). Android Police also found hints in the early November update.
But the service is still limited at this time. According to the help pages, the family group feature is only supported in six countries (Australia, Canada, France, Germany, UK, and US). Furthermore, Google's FAQ says the only current option for setting up a family group is to subscribe to the Google Play Music app first, and only manage the group afterwards.
Google says we can buy apps, books, movies, etc with that family group, although there is no explicit mention of sharing the content in the group.
In short, sharing is a feature that is coming in the near future, and is not yet available.
So let's all keep an eye out, shall we?