A book I'm currently reading briefly mentioned epistemic logic, but didn't say what it was. Since I've never heard of this logic I decided to look it up and saw that it was a type of modal logic. I have no experience with this logic either, but I'm curious to learn more about it. So before I go out and grab a book on this topic, I'd like some feedback on what I should already know before tackling this subject. I have some experience with first-order logic and I am a mathematics student at the undergraduate level. Your help is greatly appreciated.
An understanding of classical propositional logic and first-order logic should suffice. Some notes:
There are lots of other points of similarity and contrast between modal and non-modal languages. Familiarity with graphs, trees and induction would be helpful, but since you have some experience with first-order logic, I would recommend that you just jump into the study of modal logic without preparing for it. (Keep Enderton on the side if you come across topics you haven't seen before.)
van Benthem, J. (2010) Modal Logic for Open Minds, Stanford, CSLI Lecture Notes #199.