Two ePub books that I've bought won't open in Books.app in macOS Mojave 10.14. The books have social DRM (personal information added to the content of the book), and I don't think this should be a problem for Books.app. If I rename the epub extension to zip, then I can unzip the file using the unzip command. Using the Finder doesn't work somehow. Other books open normally.
Looking at the contents, I see that it uses XHTML inside an OEBPS folder, while other books use HTML in an OPS folder.
What is the reason that Books.app doesn't open these ePub files? I can read them OK with Calibre.
Best Answer
If your ebook indeed is unencrypted, you can try to open the ePub with the free Sigil.app (an ePub creator/editor) and write another copy to disk.
Sigil is able to repair a lot of structural problems. You can get a precompiled dmg as well as the full source code here.
I agree the "social DRM" should not pose any obstacle for using these eBooks with Books.app. But I suspect the "patching personal data into the eBooks" is done by an automated process, which simply went awry in your case. Maybe you even have some special characters in your identifying information (quotes, accents, ampersand, general unicode characters).