Depends on how long you want to study the craft. It's not rocket science, mate.

[ He tilts his head and looks down at the charred floor underneath them. A few digs with the tip of his shoe. Huh. ]

Arsonist would of gone for the whole building. Accident.. maybe, but, then, they got damn lucky at stopping it this far. [ The last option though. That's possible. ] Better question would be: why keep the no-no books in the library? Why not some hidden passageway?