A few pencil marks

Making a book easier to revisit.

It is possible to underline so much that nothing stands out. Try marking only a sentence that changes how you understand the page, along with a short note explaining why it matters.

For borrowed books, use a separate slip of paper. Write the page number and a few words of your own. Rephrasing an idea often reveals whether it was understood or merely sounded convincing.

After finishing, return to the marked passages before beginning a summary. A handful of useful questions is often a better record than a list of everything the book covered.