I wanted to follow-up on another thread but was unable to do so (locked after a week?). Just to provide some additional detail regarding a bugged Tome of Understanding. To recap, the tome is not usable, after being identified it only has the drop and close options.
Others in threads said they couldn't remember where they had gotten it, but I had just acquired it from the final chamber of the dark castle after defeating the last boss there. I recovered the book from one of the 'library' shelves in the final room. Perhaps it is this specific instance of the book that is the issue? I believe others stated they were able to read tomes purchased from the vendors in Caer Cyflen. I am not rich enough yet in my game to test this myself, unfortunately.
Hope the additional context helps, cheers!