It seems that most people can remember only a few high-level details of a book weeks later—if that. A typical reader might spend hours finishing some serious non-fiction—then maybe it comes up at a dinner party, and they find you can remember like three sentences. Basically no detailed recall. Barely the gist!
What’s more: people seem surprised when this happens. They seem to consistently overestimate how much they’re absorbing from a book.
See How rapidly do people forget practical knowledge?
This observation is unfortunate for many reasons, but among them: Deep understanding requires detailed knowledge of fundamentals and Complex ideas may be hard to learn in part because their components overflow working memory.
For common objections: Many people view memory as unimportant to deep creative work.
Amlund et al - Repetitive Reading and Recall of Expository Text
Matuschak, A. (2019). Why books don’t work. Retrieved from https://andymatuschak.org/books