I do hope someone will step in and correct me if I am wrong.
I believe the location of the Google Play Books directory may only be available for accessing on Rooted devices.
You could go to
https://play.google.com/books and download the .Epub files of the purchased/freebie Google Play books to save a copy to your computer.
As to sideloading your own books to your Nexus, the Google Play Books app does not read/import ebooks from the device storage. Instead you must/can upload your files at the same link I've provided previously, to Google Play Books, to store them on the cloud to have them accessible on your android devices. You can pin the book on your device to then store it on the device storage for offline reading.
For reading/importing books from the device storage, instead of uploading to Google Play Books, you would want to look to alternative ereader apps like Aldiko, Moon+ reader, Mantano, FBreader, Coolreader and the likes, which will/can read from a folder on the device that you can access to simply copy/paste to/from.