How to transfer iBook on iPad mini 3 to the Cloud?

After going to my books icon, clicking on library, selecting specific book, how do I transfer this particular book to theCloud. I've finished reading the book and want to put it into theCloud

The book is already in the cloud. All items you've purchased from iTunes, the App Store and the iBook Store are saved in the cloud and can be re-downloaded from the "Purchased" section of the store.