Archive note — October 23, 2026: This is a troubleshooting account from April 2020. The System Preferences menu names below belong to that era. For settings in your current macOS version, consult the Apple Books User Guide.
Today I was breaking my head on the fact that Books or iBooks like it used to be called, did not want to start. This happens once in a while on my Mac, with much frustration. Most posts in Apple Forums suggest deleting cookies and files in Containers and Preferences folders in your Library, like the BKAgent folder.
What fixed it for me was to check or uncheck and reboot: System Preferences > iCloud > iCloud Drive Options to make sure that Books is checked.