There are some books with the same "???" name, they contain hints on weapons and effects.
Unknown book hint: 'blackstingerstaff'
Unknown book hint: 'Freezing'
This is most likely an error of coding in the game. The game probably failed to properly display the hint. Like instead of saying: Your read how to identify scroll of light. From now on you can identify them. Instead you get the error: ??? Unknown book hint: "scroll of light"