Apple Books on Mojave stuck
Dr Google says...

Download Titles Again
If you are still having issues with the downloaded titles, be it blank pages or wrong book, delete the downloaded book.

Next, go to your account’s purchased section under Reading Now or for iBooks, visit your account settings in the iBooks Store and re-download the title.

This likely solves your issues even when you have partially downloaded Books.

Also see:  https://discussions.apple.com/thread/4421355 for a similar fix.