This supplement presents an interesting idea for determining whether or not a character knows something about a given monster type without introducing a slew of new mechanics to accomplish such. However, the mechanics are a bit inflexible and a little illogical at times; essentially, every class can add their proficiency bonus to Intelligence checks to know something relevant about a particular monster, but as the other reviewer below noted, each class is locked into knowing something about a particular type of monster. Rangers, for example, can add their bonus to checks about monstrosities, and Barbarians can for beasts. It seems a little odd, though, that a Ranger seems to have no particular knowledge about beasts or giants. In attempting to make each class have its own particular field of knowledge, monster lore ends up getting divided up in ways which seem forced and unlikely. I think it would be fine for some classes to be better than others at such and also for some overlap to occur.