I checked several sources on this. Googles own help page states:
The Google Play Books app automatically stores books in the location with the most free space, whether that's your device or your SD card, as of when the app was first launched. If you've upgraded your app, your books will continue to be stored in the same location they were before the upgrade.
Which is not very precise. A discussion on the Archos forum reveals a setting available in the app (Settings> Storage location), but there you can only chose between internal and external storage. But the same thread as well as another one on XDA reveal the path as being
/data/data/com.google.android.apps.books/ (which is the apps data directory on internal storage), the Archos forums thread also mentioning
Android/data/com.google.android.apps.books/files/volumes/, which could apply to the "external storage".
As I don't use Google Books, maybe you could check those locations and confirm.
Note that accessing
/data/data/* requires "root powers". Without that, you probably won't see anything in that place.