You can save your book to Google Drive, to share it with other people.

To do that, tap on the three-dots options menu-button.

If you have Google Drive installed on your tablet, you can now tap on „Share“ to save your book to Google Drive.

Now tap on „Save to Drive“.

To save and open your book in Google Play Books, tap on „To Play Books“.

This will automaticaly save your ebook in your Google Books library.