You reach the end of a chapter, turn back two pages, and find a sentence you marked without quite remembering why. That small surprise is part of the value of annotating. A note in the margin can preserve not only what a book said, but what you were thinking when you met it.
Mark questions, not just favorites
Underlining a beautiful sentence is satisfying, but questions often make better companions on a reread. A brief note like “Who benefits from this rule?” or “Why does this feel different now?” leaves room for your thinking to move. It also helps keep annotation from becoming a contest to collect the most quotable passages.
Choose a light system
A pencil, a few page flags, or a note on a separate sheet is enough to begin. If color helps, give each color one job, such as marking questions separately from recurring ideas. A complicated code can pull attention away from the book; the point is to make it easier to find the thoughts worth returning to.
Let the marks change
When you revisit a book, your earlier notes are part of the reading, not the final word on it. You may agree with the old margin comment, cross it out, or wonder why it mattered so much at the time. Try leaving one new note beside an old one. The difference between them can tell you as much about your reading life as the book itself.