Hi marcjbannes,
welcome to this Apple (user-to-user) Community!
Re: "Prevent Books from offloading files on iPad Air" : ... Certainly there is a way to toggle this off and prevent frustrating offloading when using the device to read in a location without WIFI."
See Apple support's online article:
About purchasing movies, TV shows, music, books, and ringtones from Apple - Apple Support
Eg: says:
“Any digital media you buy from Apple can be permanently downloaded on to an iPhone, iPad, Mac, or Windows PC.”
and
“Once downloaded, you can access content without an internet connection, and Apple can’t remove it from your device.”
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Re: "I don’t want my iPad Air to offload files from Books".
Wondering whether you may be short on memory / storage space for smooth operation of your iPad, as well as for storing all your Book files. If so, it is wise to back-up your iPad before making more space available.
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If needed: See the article: How to check the storage on your iPhone and iPad - Apple Support
or
Apple Support also have a video online, that can guide you to check storage, and make more space.
How to free up storage space on your iPhone or iPad
https://www.youtube.com/watch?v=bwjcYyCneNc
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If needed: See how to : Redownload books and audiobooks
All the best :-)