Tunnels/Fires of the Black Book is listed as underground. Is that a mistake? The only way I could think of to test it would be a scuffed mining helmet, and that wouldn't make much sense.
Edit: Al pointed out that the bonuses page would still update your stats. Someone should try the helmet though. And also (another idea of Al's) try fighting an enemy like the tin soldier. Doesn't seem logical that it should give you the underground bonus for fighting in your apartment with a book open nearby.
In the beginning, men were mortal.