Adding to that: Write down what you want to get out of the book, for example "Learn how to do X" or "Understand how Y works". This turns passive reading into actively looking for information.
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".
I asked this mostly for self help books, like time management and others. Obv you would need to remember and act what was discussed in the book to get any benefit from that.
You're supposed to read these books multiple times. You read the first time to get a general understanding. And then you reference things within the book as they pop up as ideas you want to remember more about. No one remembers the details of anything they read the first time. It's about constant exposure. Some things however, are important for shaping your general thought processes, but not important enough to remember specifically
Why read a book if you're not going to remember it? If you want someone to tell you what to do, that's what articles and tweets are for. Books are meant to go into depth.
Books are the most efficient way to experience other people's living and learn from it. You don't memorize your past experiences just like you don't need to memorize a book to get influenced.
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.
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".