It sounds like the system is checking total level and not wizard or druid levels individually. As such a level 18 or so character (who is only level 4 in wizard) is given the book and can use it. I will be fixing this bug so that it works as intended when it was designed/created.
Anybody that has a tome already, that does not meet the requirements as they have been specified, will have the tomes removed at character login time.