It appears the easiest (most familiar to me) way of moving files from Linux to iOS will be by using the NextCloud server I host for my family/friends. NextCloud is like a DropBox type of thing, except I host and control it on my own hardware and have complete control over all my data, stored in my own databases, handled by my own secured web services.
I just need to install some WebDAV client software on the iPhone to connect to my NextCloud server (or I could use the built in Safari web browser). I found "Documents, by Readdle" (a.k.a. "Documents 6") to be one such WebDAV/filemanager available for iOS. There are others. "Documents" has a built-in music and video player, and this may work for audiobooks. I'm assuming I would need to set up a playlist to get all the individual MP3 files that make up an audiobook to play seamlessly, in order. Remembering where in the audiobook I last was might be an issue however. I'll play with it and see what type of memory/bookmarking it supports.
I just wish there was something like Androids "Smart Audiobook Player" available for iOS. That's a great app.
https://itunes.apple.com/us/app/docu...364901807?mt=8
https://nextcloud.com/
https://play.google.com/store/apps/d...layer&hl=en_US