I recently updated to High Sierra, and am now unable to open books (ePubs/PDFs alike) via iBooks. These books are there, I can drag them out to my Desktop or sync them to my iPad, but they do not open in iBooks.
Instead, an alert is displayed with the following text:
The operation couldn’t be completed. (BKAgentServiceErrorDomain error 8.)
A black-HUD-ed spinner appears on the book's icon, spinning indefinitely.
How do I resolve this issue?