Cannot start iBooks error
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 BKAgenent 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.