Not sure, but you might look at Calibre on the Mac.
https://calibre-ebook.com
I know Calibre can maintain a library on your Mac, but what I am unsure of is how you transfer ebooks from Calibre to iPhones, and what ebook reader you would use on the iPhone.
I have only used Calibre to track the books I read, and since I DO have internet access, I just allow Kindle to fetch the ebooks for me.
Second, there is an iPhone app called "Shubook M2" in the iPhone App Store that can download ebooks from different sources. The ebooks need to be Digital Rights Management (DRM) free, which I assume the books you are collecting are:
https://apps.apple.com/us/app/shubook-2m/id950017941
I'm sure there are other ebook readers available for the iPhone (and most likely Android phones). I just know about Shubook M2.
Finally, I know that libraries in the U.S. have access to specialized ebook lending apps. The library buys an ebook with 'n' concurrent distributions, and the software will only allow 'n' library patrons to read the ebooks for a 2 week period, and then the ability to read the ebook expires and another patron can read the ebook. I only know this software exists, and that publishers have a method of charing public libraries that lend ebooks this way.