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: Conceptual understanding is unlikely without detailed knowledge of fundamentals and Complex ideas may be hard to learn in part because their components overflow working memory.
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