The "Form of Frog" spell was cast on an explorer who had the book (+2 to knowledge, if you loose the book -2 to knowledge).
The Form of Frog spell states that when it effects and explorer it drops their might and knowledge to the lowest level above the skull. It also states that they drop all their items meaning they take the -2 knowledge hit from dropping the book.
I would do those two things in reverse order.
Drop book. Lower knowledge by 2. Then turn into frog. Go to point above skull.
If losing the book would kill them, it kills them. But turning people into frogs shouldn't kill them right away.