For anyone interested in a deeper (but not dense) introduction to how and why Feynman diagrams work I’ve found a lot of pleasure going through this blog post below.
Feynman's own book 'QED: the strange theory of light and matter' is another good read, but strangely it is a bit lacking in illustrations. (And the illustrations that are there are a bit dry compared to Mattuck!)
https://www.quantumdiaries.org/2010/02/14/lets-draw-feynman-...