My goto in this is to read the book, and when something relevant comes up, stop.
Think on whatever it is and run through related experiences and model possible, alternative outcomes, given whatever insight is at hand.
The way to remember in an enduring way is to make connections. For that to happen, you must invoke what you know.
Simulation and extrapolation are two great ways to do this.
Affirmation, as in "that's why it happened as it did" is good too, but is also easily forgotten.
Extrapolation and simulation make much deeper, enduring connections, and those are what you seek. With this effort will come a genuine change in, or expansion on your perspective. There will also come decisions too. Perspective changes are not always a net positive. You need to give the insight enough consideration to judge this.
Extrapolation can be near future events too. Model them, get your expected outcomes and what drives them clear, then refactor with that insight. Compare to your past or others experiences where possible.
Does it make sense outside the context in which it was presented? Does it contradict that which you hold dear? Why? Is it self consistent? All of these and more will play out for you during this investment of time and thought.
It can be very helpful to do this with others too. Group model, simulate, extrapolate then reassess what makes sense.
Most of the things in those books come down to a handful of words, phrases, ideas. The book is there for context and comprehension, as well as drama to get you through to exposure.
Your own context is where the value is, and the work to realize that can often be done on a live chat with friends, standing in line, at the bar, etc...
Realization does take some human time. That is where the really good stuff is.
My best improvements have come from these activities and some new ideas to process and understand what they mean in my context, not just the context in which they were presented.
Don't pick up the book again, until you have processed these things. A very good indicator is a sense of new motivation or urgency related to something you normally would take as it comes. When you feel that, continue, and it should become more resonant, and when it does, consider action then.
Doing this is also a great bullshit or flash in the pan filter. Shiny things can cost us. Should they remain shiny after handling them for a while, chance of real value is improved.
So handle it. Try to rub the shine off, pick at the seams, find the edges, poke, prod and work to see what it is you have really got.
You won't forget that so easily. You may also find connected thoughts and ideas too. These can have more value than the insight.
I will carry one of these ideas around for some time. Quick is not always good. A genuine insight can take some time, days, maybe weeks even, to play out.
Of course, this does then bring up how to understand what is a waste of time and what is not.
No answer for you there. There is genuine risk in all things. You can abandon a book that does not yield net improvements, but only after internalizing enough to understand.