I've been doing this thing recently where I'll make a list of the chapters in the book and every time I finish a chapter I'll write a small summary and even highlight it a color that feels appropriate if I feel it would help me remember it better. I'll show you what I mean.
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This series had 5 books so after every chapter I tried to summarize it as succinctly as I could.
What I noticed is that this helped me memorize so much more than I usually do about a book. Because I'm always coming back to the list as I progress thru the book, I spend more time looking at how the story is structured, how the pacing has been, and where I think the story might go next.
Plus if I ever in the future forget details, I'm sure looking over this will quickly refresh me.
Do you take any notes? It's obviously not necessary, but it seems to help.
I use Kindle, and it has a great feature of highlighting text you want. Every time I see a worthy passage I highlight it. Usually it comes to 1-5 passages per book.