BKAgentServiceErrorDomain error 8 This error is shown in a dialog when you're trying to open a book. The dialog appears and you get a spinning HUD.

"BKAgentServiceErrorDomain error 8"

Today I solved the problem together with the Apple Support. I couldn't answer the original question (not enough reputations), thus I created a duplicate.
The solution that worked for me:

1) Start syncing you books library via iCloud

2) Close the books app (macOS).

3) Navigate to your ~/Library/Containers/ folder.

4) Move the folder com.apple.BKAgentService to a save location.

5) Restart your computer.

6) Start the Books application.

7) Wait, the books will be downloaded.

8) Open a book (it may take a while, because it is downloaded)

If you did not synchronize via iCloud then you have to add manually your books.