The Physiologus is an ancient tome handed down from one village wise woman to the next. It contains descriptions of all the monsters known at the time it was written — which is not to say it is not comprehensive, and how to deal with them. Geralt obtains a "Book" as part of the reward he receives for "dealing" with the deovel that was bothering the village of Lower Posada in the short story "The Edge of the World". That "Book" seems to be a copy of the Physiologus.