I'm not sure you can actually share a whole book with anyone. You can share a bookshelf, which means others can see what you have. I think they'll be able to read a sample of the book, but they won't be able to read the whole book. The Google Play Books user policy explicitly states, "You may not lend or co-own any of your Books on Google Play purchases with another person."
https://support.google.com/googleplay/answer/1062968?hl=en