What you actually need is knowledge of an overview of the Mathematics of Formal Methods in all their aspects viz. Predicate Logic, Set Theory, Temporal Logic, Type Systems etc.

Towards that end I highly recommend the following books;

1) Introductory Logic and Sets for Computer Scientists by Nimal Nissanke - An easy-to-read book needed for background mathematical fundamentals.

2) Understanding Formal Methods by Jean-Francois Monin - This gives an excellent overview of the core topics and a must-read. From here you can branch off into studying any specific approach and its tool (eg. Lean4/TLA+/etc.). Once you have studied this there is nothing "Formal" (specification/verification/etc.) which is impenetrable.

3) The Correctness-by-Construction Approach to Programming by Derrick Kourie and Bruce Watson - Walks you through the entire process of deriving a program using successive refinements from specifications using Hoare/Dijkstra approach.