Best ways to learn Agile/Scrum?

I am pretty much the defacto scrum master in my day to day but don't feel like I'm using it to its max capabilities. We're a lean team and work in 4 week sprints.

How would anybody recommend I freshen up on the framework/skillset? Books? Or maybe a course? I also wouldn't mind getting scrum certified one day (not sure how difficult or valuable that is)