BKAgentServiceErrorDomain error 8
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.