I've found the importance of remembering a book you've read depends on why you're reading the book in the first place. If you're reading to entertain, remember

I've found the importance of remembering a book you've read depends on why you're reading the book in the first place. If you're reading to entertain, remembering what you've read isn't nearly as important as the experience you get reading it. It's why I've reread so many of the science fiction and fantasy books I love. Reading a book again, especially long after the original reading of it, re-ignites the pleasure of the experience of the first reading. The third and fourth levels of reading often require the reader to do exactly what you describe. I've actually started using paper index cards to do this, creating physically cataloged notes, which through the muscle memory of writing, help to embed the most important concepts and idea in my brain. I personally read a lot of books that inform or advance important ideas casually first. Then I go back and re-read sections, taking notes more carefully to properly ingest the critical topics into memory.