If you go into the Character menu and look under Attributes, then Knowledge, there's entries for all the different monsters you've learned about.
I know you can learn these entries via Books (level 3/3 knowledge right away) or by killing enough monsters (gradually increases to 3/3).
What I don't know is the value or benefit to this knowledge. Increased damage to those enemies? Increased drops from those enemies (and if so, is that required for certain special craft components) ? Is it just the advice to the player (like don't try to poison Endregas) ?