A good book often invites us to reread. The best books often get reread several times. As I go back to a book that I read long ago, I am always encouraged at how much of it quickly comes back, as I begin rereading.
Underlining books as you read them will impress the thoughts you have picked out on your mind but it will also aid in how quickly the information comes back to you. Underlining in red or yellow is a favorite approach but even a lead pencil is better than nothing.
Books that have been underlined offer another benefit in that you can reread just the underlined parts and it is almost as good as rereading the whole book. Sometimes I think this is even better than reading the who book because the focus is on what was already determined to be important.