Are book samples backed up via iTunes sync or iCloud?

A friend of mine has a couple dozen book previews on her iPad. She downloads them so she can read the sample and purchase the full book when she's about to travel. Recently we did a restore on her iPad and found that while we had done an iTunes backup, the full books that were purchased returned to her device, while none of the book samples did. Is this resolved in El Cap or later? Or is there an existing way to backup this content? Right now book samples don't ask for Apple ID credentials, so I'm wondering if none of this material gets backed up, and therefore there's no way to recover it.