iPad memory full. Can I upload books already read to iCloud to free up space?
iPad Mini version7.1.2
Memory completely full. What's best way to increase usable memory -- upload books and music to iCloud or do I have to delete? This
You can't upload purchased content to iCloud, because it already exists in iCloud. You should be able to delete the content and download it again later. However, if the content is pulled from the stores for whatever reason, it would not available for download again.
Audiobooks can never be downloaded againfor free. You have to pay for them again. If you sync with iTunes and transfer your content into your iTunes library, you don't have to worry about content not being available in the future since it would now be stored on your computer.
Download past purchases from iTunes in the Cloud - Apple Support