There is reading for pleasure, and there is reading for learning.

The latter seems to be more like what you are asking about, and it is a more time consuming process compared to light reading. I've read tens of thousands of books at this point and remember most of the important bits (decades later).

It is a skill that requires some discipline as many things you will be reading about you will necessarily, not agree with, nor should you generally agree without significant vetting.

I haven't heard of a formalized process for what I do but many others with similar experiences seem to have reached this same process by parallel construction.

The gist is, you read things multiple times and you do different things after reading that have a synergistic effect. These things don't take a lot of time or effort, its mostly do it once wait some time do it again, revisit. You don't focus too much, just enough to remember; you want to eventually practice getting into a state of relaxed focus with precise control.

If you've ever played an instrument, you'll have gone through a lesson where you learn when you tense up you have less control, the moment you relax that control is much more precise. That is what you will eventually aim for, relaxed precision.

Every person forgets things on a fairly regular time curve that lengthens with each successful recall. This varies greatly between visual and auditory, and at most you can hold only 7 related connections in your head at any given time, aim for 5. There are various strategies you can use such as the Cicero memory technique (Mind Palace), or the Russian doll method of memory technique. These advanced methods aren't for everyone.

https://en.wikipedia.org/wiki/Forgetting_curve

Remembering any complex subject usually involves a three stage process, and depends on the material being consumed.

Anything requiring a high amount of discernment and critical thinking (i.e. convincing fallacy/misleading lies/anything related to communism), should never be read hurriedly. There are structures that naturally bypass critical thinking, and back-tracking to re-read is entirely normal.

Speed reading also bypasses much of our critical thinking faculties and you don't want to accept something as true when its not, you run into more trouble unlearning things, and don't have the ability to recognize these things going at that speed. Exceptional speed readers may even turn written words into a film in their heads. Practicing to not speed read material under certain circumstances can be very difficult, but it provides many benefits and avoids pitfalls.

The following process for learning from reading can take a bit of practice if one has gotten into bad habits, or has never developed good habits because of learning/visual issues such as with dyslexia.

First step is you read through a chapter as quickly as possible. You segment into broad concept and detail, and you discard any details not wasting effort, your focus should be on how the main subjects connect to each other, or how they are associated. Its all about getting the structure of the overall context together.

Musk does the same thing but mentions it as branches and leaves. You won't be able to remember details if there is no context you can hang those on (or walk down).

Many other extremely intelligent people have discovered this process, it predates Musk.

When you are done, and have focused on the main topics to retain them, take a 20-30 minute break where you don't think about it at all, relax do something else unrelated, set an alarm if you have to. Then come back to it.

Try to recall what you just learnt. If you are successful, you mentally walk through it again an hour or two later, then 4 hours, you should have a good chance at remembering it the next day, and you do this for several days, then a week, by 3 weeks you should have it fairly permanently so long as you recall it every couple of months (it becomes easier).

Importantly, more time and over-practice doesn't really help, so long as you can remember it when prompted you can do anything else you want.

If you are having trouble coming up with the structure, think about how you would explain the concepts of a chapter like a paraphrase or summary of what's ahead that you need to pay attention to if you were asked to tell someone who hadn't read it, but no details.

Second read is all about the details, and how they relate to the context. You can do this alongside context, but it works better if context is developed firmly first (you can do more or less in parallel just not the same or related material, you'll notice its more difficult when its related because of destructive interferance; best example is when trying to learn different languages in parallel). You should encode what you learn in a positive form. Negative forms end up interfering destructively. A negative form would be something like do not do this, or do not not do this. Simplify it in a positive. Do this.

If you need to force yourself to forget something intentionally, by all means create a negative form and do the same process. Its the quickest way of unlearning certain things with a little practice.

This second read, you should focus on the details, and how they are connected to the context. Instead of memorizing the details, synthesize and paraphrase what you have learned and connect that with the main topic.

For truly dense books, you won't pick everything up without multiple reads.
On the third read, and beyond include a list of questions you might be asked or ask yourself, related to the concepts covered. This develops intuition and flexibility so you can approach this subject matter from other related concepts which usually start in the form of a question. You fill in the gaps connecting the material. Each time you learn something new, you follow the basic recall process on the forgetting curve timetable.

After a time, with sufficient intelligence, associated information just comes unbidden. When you recall one thing, you tend to refresh the related others, eventually the nerves in your brain grow together and its mostly permanent recall.

Intelligence is at its core speed of thought, and wisdom is a combination of intelligence with memory that's been tempered by intuition.

Having a clean diet and exercise can also greatly impact recall and learning of material learned through books. You will be amazed by some of the useful things you'll find in books published in the 70s and prior.

To be able to think, one must risk being offensive.
To be able to learn, one must risk being offended.

I hope you find this useful.