iBooks can't access my library

iBooks can't access my library

When I try to launch iBooks on my iMac I get a message: iBooks can't access your library, with the explanation (?):

"Make sure the disk containing your library is connected, then click Try Again.

Library Location: (null)

You can also reset your library, then sign in to view and re-download your purchased books."

There are three buttons "Quit, Reset Library (grayed out), and Try Again"

Clicking any but Quit is useless. I have no idea how to revert the library even if I knew where to find it.

Please help.

as per the previous reply to this question,

i tried trashing my plist and restarting, but that didn't help.