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.
Those books are typically stored in "system" space, with the app. Which means you can't see them, unless you are rooted.
everything I read about rooting says not to root it unless you know what you're doing