It is quite possible to read 100 books and retain knowledge. I am at book 86, and it is only July.

I walk a lot, so listen on Audible. If there are charts, I check them out, sometimes purchase a physical copy. The idea is to read multiple books about the same ideas from different angles. This way, it is structured, and there are bound to be repetitions. Reading about similar ideas from different people helps consolidating them, as well as critically analysing them. Make sure to throw in an author that you know you won’t agree with here and there, and voilá, you’ll end up with a pretty solid understanding of the area you are interested in.