Books in the Cache folder?

The app asks me to restore my purchases every once a while, especially when I have too little space on my iPad. After some research I found that my purchased books are stored in the cache folder and it gets removed when iPad needs more space.
Developers, do you know what cache means? Put the books in a new folder in the Preferences or put them in the Documents folder.

Buggy Simon about The Smurfs Classic Series