I copied this section from: http://www.igeeksblog.com/how-to-export-ibooks-from-iphone-ipad-to-computer/ I am not sure how much of it is useful to you.
1). Get a file explorer. Usually, iExplorer should do just fine.
2). Connect your iPhone or iPad to the computer and fire up iExplorer
3). Open your iPhone/iPad as a mounted device and navigate to:
Media → Books → Purchases
The funniest thing is that you won’t be able to decipher what file corresponds to what book from your library because, like music, the file names are encrypted. Nevertheless, you can copy stuff now.
This folder method gives you access to all files including .epub, .pdf and all types you have purchased. You might want to check out epub to pdf converters to make a readable copy of the epub iBooks you’ve got.