It also helps to work with the ideas in other ways while reading (like taking notes, creating presentations, or writing programs).
I'm currently reading about production scheduling and in parallel writing a toy production scheduling system to make the concepts "click".
It's kind of a circular definition, but the parts that are important enough to me to remember are the ones that I remember.
Now, if I'm trying to get some specific information, I will often write it down somewhere when I find it.
If it's something like language syntax, the pieces of syntax that I use all the time are the ones that I remember. The ones I don't, well, I know where the book is, and if I need that bit, I'll look it up when I need it.
For non fiction that is usually obvious. If there are not set exercises then make some challenges up.
For fiction. Maybe a character map? Or make a list of characters and notes against each.
Chimpanzees, for example, have photographic memory. They remember absolutely everything. Photographic memory is not likely for corvids but their memory may well be superior to humans as well. In that regard they are more intelligent than humans but lack the tools in their toolbox to achieve superior mechanical or functional output.
Back when I was making gears I had a set of 3 of the books in the late Eric Flint's 1632 series I read in rotation over lunch breaks. It was slow enough due to the limited time that I'd have forgotten things enough to make it interesting again each time.
This is why churches teach the same lessons every year in a great cycle, the liturgical calendar, to get them to stick .
2. I paraphrase and write down the most important passages in my notes. Paraphrasing is important for memorization and understanding for me.
3. I go back and read highlights after some time and repeat the process.
But I end up retaining some associations, a kind of mood, and I also tend to grasp the tendencies and attitudes of the author.
So in a way I read between the lines a lot. I am able to read with a kind of generosity of spirit (forgiving, listening, giving charitable interpretations, etc)
This way - I end up forming the most profitable kinds of memories - something that may help me live my own life and solve problems of interest better.
Reading transports you to a different reality than your present (in psychological/experiential dimension).
Which is very important for invention and such.
I wrote about it here: https://rishikeshs.com/readwise-review/
Multiple books by an author if it’s fiction/poetry works similarly but that’s more about deepening my appreciation than recall.
Additionally, I don't worry about trying to remember every little detail. I also don't really take notes.
I draw a flow chart of key words as I'm reading. This does two things: 1) It keeps you active, which helps prevent your mind from wandering while reading 2) it gives a roadmap of what you read. Once you filled a page with your flow chart, stop and visit each node and recite how much you're able to remember. Star the items you can't recall and either go back and review that material or move on and come back to review later.
Part of what I'm realizing creating the flow chart does as I'm writing this, is it helps you differentiate between what was easy to remember and what was not. Also, don't try to get fancy with the flow chart, just draw basic circles (nodes) and directed edges. The flow chart should be a DAG, but avoid branching as much as possible.
* Re-read the books
* Summarize the book in one sentence or a short paragraph
* Create or visualize the idea or important points in the books, like https://sketches.sachachua.com/static/2022-08-03-01%20Four%2...
Last but not least, I recommend to read a book called "How to Read a Book" by Mortimer J. Adler. Different books have different strategies to read.
* Write a short summary of a chapter after reading it
* Don't be afraid to to write notes and highlight important lines in the book if you own it. You own it so make it yours! If you do want to avoid marking it up 3m makes some nice transparent post it's that I find work well (https://www.post-it.com/3M/en_US/p/d/cbgbjrus3149/)
* Before reading the book do you have a specific reason for doing so? If so look at the index at the back and read the sections that are specifically relevant first so that you will have some repetition reading the content.
For me personally I read a lot of non-fiction and like to have the books as reference material so 75% of the books I own are physical. When I read them I highlight important / relevant passages to me and possible make notes in the side margins if I find it relates to another book I have read and remember.
After finishing the book I'll eventually type out the relevant lines I found then and put them on my personal site for easy referencing for my self. This is doubly helpful as it means I'll often re-read the book down the line further reinforcing what I've read. I do have quite the backlog for this but it equally means that if I need to go back looking for a specific snippet on something I likely have it highlighted
I'd also second the recommendation for "How to Read a Book" by Mortimer J. Adler https://www.amazon.ca/How-Read-Book-Mortimer-Adler/dp/067121...
Most of the non fiction books are watered down garbage anyway. And professional ones you read when you have a specific need to apply new knowledge.
And fiction ones you read for the pleasure.
There is no point in forcing yourself to memorize a book. Unless you naturally do it or feel like doing it.