There's a certain gap between reading and retaining. It may be personal. One may find a newly read knowledge, concepts illumintating and clear at the time of reading them, yet after a short time that clarity vanishes. Like as if it somehow failed to transfer from the short-term memory over to the long-term index of the main library. The only lasting recollection is of experience reading it and feeling illuminated. Happens to me everytime I need to cram something quickly.

I believe minds do need reread, retry in order to retain it as something worth recalling. I guess, some brains prefer not to waste energy on forming the long-lasting synapses on something that was casually encountered.

The book "How to Read a Book" comes up here occasionally, and this is (I think) the second level of their 4-level system, after examining the table of contents and end matter.

I may have gotten it from there, tbh. That first step looks familiar.