Physically clearing the data from Google Play Books in the Settings>Apps menu is not the ideal way of freeing up space. Clearing the data means you also clear out the app's database, so it may be having trouble accessing your Google Play Book library in the cloud. If you want to remove a book from memory, the best way is to tap that pushpin icon at the lower right--you might have to tap it a couple of times until it gives you the warning that you're removing the book from local memory, but that you can still download it from your library.
In terms of accessing your library again, what happens when, while in the Play Books app, you tap Menu>Refresh?