I'd like to add an addition to A.K.'s otherwise very good answer to address the topic of progressing to the "state of the art." A.K.'s reading list will get you the basics. That will be very doable on your own. By contrast, it's very difficult to progress to the state of the art without a mentor or PhD advisor, simply because you won't have any orientation or understanding of the basic trends. If there are any logicians at your university's philosophy department, even if you cannot study under them formally, I would recommend meeting with them and seeing if they'd be willing to give you any guidance.
In any case, after you read Fitting and Mendelsohn and get a basic grasp on modal logic, I would recommend reading Holliday's work on epistemic and doxastic logic as an excellent guide to the state of the art today: Cf. https://philosophy.berkeley.edu/file/856/EC-EL-1.pdf
Beyond that, you will need to get to a level where you can read and understand the significance of the articles published in top logic journals. I would recommend places like The Review of Symbolic Logic, the Journal of Philosophical Logic, Studia Logica, The Notre Dame Review of Formal Logic.
I would definitely not recommend reading Kripke and Lewis if your goal is to understand the state current epistemic/doxastic modal logic. That will send you very far astray from the problems that logicians work on, and into territory that is resolutely philosophical.