I'll take a step back, why do you need to remember what you have read in a book?
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.