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.
I can’t find real empirical data on this question, so this is largely an anecdotal claim. Some light evidence in suggested by Amlund et al, who showed <50% free recall minutes later, even when readers knew it was a test.
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, J. T., Kardash, C. A. M., & Kulhavy, R. W. (1986). Repetitive Reading and Recall of Expository Text. Reading Research Quarterly, 21(1), 49. https://doi.org/10.2307/747959
Matuschak, A. (2019). Why books don’t work. Retrieved from https://andymatuschak.org/books