- I feel if someone explains a concept you do understand (reflection) in such an unhelpful manner, you should not count on them to nicely explain concepts you don't know.
There's a lot of good stuff in that book *if* you already have the main ideas. I humbly recommend starting with my lectures on Dynkin diagrams, or my lecture notes, here:
https://math.ucr.edu/home/baez/twf/index.html#dynkin
