To get an overview of what all are involved in "Formal Methods" see Understanding Formal Methods by Jean-Francois Monin. The book gives an overview of both the various mathematical models and some of the tools implementing them. There is a lot here and it may seem haphazard but that is only because we haven't yet grasped the "full picture". I have been reading this for a while but still have a long way to go.

A four-part TLA+ in Practice and Theory by user "pron" - https://pron.github.io/posts/tlaplus_part1

Not the OP, but Hillel Wayne’s course/tutorial (https://www.learntla.com/) is fantastic. It’s focused on building practical skills, and helped me build enough competence to write a few (simple, but useful!) specs for some of the systems I work on.