Your brain isn’t a hard drive. Taking notes will prompt your mind for a memory of what you learned, though it’ll never serve as a full catalog of a book. You’re not only going to forget stuff—you’ll actually realize how different the things you noticed were when you first read the book. What stands out to you now will be different from what stood out before.
That’s the beauty of rereading books. It’s not a waste of time; you’ll be able to gain depth in familiar passages, and connect entirely new dots with your new life experiences. In some ways, it’ll feel like reading an entirely new book.