You finish a book, recommend it to a friend, and then struggle to remember the part that made you recommend it. Or you return to a half-read novel and cannot remember when you last picked it up. A reading log gives those loose details a place to live.
If you are wondering how to keep a reading log, start with a small record: the book, your current place, the date, and one thought worth keeping. You can use paper, a spreadsheet, or an app. The useful system is the one you can update without turning the end of every reading session into homework.