Apple Books on Mojave stuck I launched Apple Books on my 2016 MacBook Pro (10.14.5) to continue reading some book I had started on my iPad Pro (where it was purchased). 
As expected the book was in the cloud so I double clicked its cover to download it. It prompted me for my Apple ID and password, as expected and then it started to download 16 other titles before this one, completely freezing the app. 

The download is not progressing, nor I can stop it. Relaunching the app, or even after a reboot, gives me a Library full of greyed out items without any chance to force a download... 

What can I do to make it unstuck? 
I even went in Preferences and unchecked automatic downloads of purchases.
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.