I read a minimum of 100 books a year. I re-read important books often so sometimes within that 100 books is a book I've already read once, twice, or thrice. When I read them I always have a highlighter rather than a notebook. Highlighting helps me to remember for some reason key things even if I never go back to the highlighted section. Perhaps it's the physical act that helps. Then as I'm reading I am constantly rewording and explaining the book chapter by chapter and concept by concept to others. My wife, my classmates, my friends. And I think and ponder them deeply for the day, which I'm sure is natural when you consume content be it a movie a book a sermon etc that you ponder it. It is for me now after making it a practice for years anyway. This helps me to retain a lot of the information I read and grasp it at a deeper level. I think that coupled with the re-reading of books is the best way to internalize a book and be in continual conversation with it so to speak. I am always reading several books but not at once. I will progressively reading sometimes 5, 10, 20, 30, generally around 10 books, and will read one for a few days and then switch and cycle through them. This I think is entirely subjective though. I could not read as much as I do now if I read only one book cover to cover at a time, I was never able to do that when I was younger and still cannot.