Hi lindaokeeffe,
welcome to the Apple (user-to-user) community!
Re: Cannot read a purchased eBook in a different region : I cannot seem to access the book
Wondering whether or not you are still seeing the book on your Mac. What happens when you try to read it ?
or
Perhaps the book may be discovered on Mac via a Finder or Spotlight search, or by revealing "hidden" files.
Eg: Hide and unhide music, movies, TV shows, audiobooks, and books - Apple Support
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Many of us may keep archives or back-ups of our Mac / iCloud files / purchases / etc:
eg: from Mac you may have saved files to a jump drive, external hard-drive, or used Time Machine to back up files externally. If so, you may be able to recover the book that way.
Else: If you haven't tried already: Wondering if you can still re-download the book:
(Sorry I haven't tried this myself, with books purchased in another country. Some countries do have restrictions)
How to: Redownload books and audiobooks - Apple Support
and What to do:
If there's an issue when you redownload apps, movies, or other content from Apple - Apple Support
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Also: If you wish: You can send feedback to reach those who work behind-the-scenes on the Books app:
Feedback - Books - Apple
All the best :-)