I wish to learn about Epistemic Modal Logic in a way that's rigorous yet easy$\color{red}{^1}$ to understand. Could you recommend books, courses, or any other relevant resources I could look into?
I have a background in Set Theory, Propositional Logic, First-order Logic, and Basic Modal Logic (I am familiar with the basic modal language, models, model constructions, frames, completeness, etc. My primary reference for this has been Blackburn's Modal Logic – the first four chapters). Happy to provide any other detail necessary.
$\color{red}{1.}$ By "easy", I mean something suited for undergraduate math majors. This isn't a hard requirement though – I am open to all kinds of recommendations!
P.S. Since this question is really specific to Epistemic Logic, in all probability it is not a duplicate.
Best Answer
Since you mention that you are OK with the first four chapters of Blackburn's et al. Modal Logic, then, technically, you already know Epistemic Logic (EL), which is essentially S5. A classic textbook on EL, which includes various notions of knowledge, completeness and complexity results, etc. is Reasoning about knowledge by Fagin, Halpern, Moses, and Vardi.
For continuation, I would recommend Dynamic Epistemic Logic (DEL) by Hans van Ditmarsch, Wiebe van der Hoek, and Barteld Kooi. It fits the description of being 'easy yet rigorous' perfectly.
For an overview of topics in EL and relatively recent developments, Handbook of Epistemic Logic is a nice option.
But, tbh, if you tackle DEL, then going straight to papers is the best.
Edit: also, for gentle introduction, I would suggest the corresponding chapters of Modal logic for open minds by Johan van Benthem.