itunes and iphone compatibility request

iOS & iPadOS

Hi all:

This seemed to be the best place to post this. So here's my issue.
On my IPhone 5s I have the Read To Go bookshare app, on which I have something like 4 or 5 hundred books. Naturally, I'm working my way through them but it's a long fought battle.
However, my Iphone is having some issues, namely the storage issue that has been reported a couple times. Basically I have pretty much no storage, yet the total data taken by apps under storage in settings doesn't equal the amount of data used. Itunes on the pc doesn't record any additional information either. I've done the trouble shooting involving cleaning apps, deleting stuff that would fall under "other," and so on. No luck. So I need to backup and restore my phone to the factory settings and hopefully solve the issue.

But, I really, really don't want to have to write down and then redownload 500 books. Is there a way to backup Read To Go books to a pc? It doesn't seem to fall under filesharing, nor can I find it listed anywhere in the itunes folders. In theory it's saved data so I should be able to store it, but I can't for the life of me figure out how.