"The operation couldn’t be completed. (BKAgentServiceErrorDomain error 8.)" 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?
This is crazy, but it worked (after everything else failed). I opened:

~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Updates


And clicked on the ePub/PDF files there. iBooks opened them. In my case, I had 1 ePub file, and 1 PDF file. I don't know how or why, but I can confirm that it is an issue related to the Updates folder, because simply deleting that folder caused the problem to resurface.

To all those out there - good luck dealing with this infuriating app.