Why is marking a book indispensable to reading it? First, it keeps you awake — not merely conscious, but wide awake. Second, reading, if it is active, is thinking, and thinking tends to express itself in words, spoken or written. The person who say he knows what he thinks but cannot express it usually does not know what he thinks. Third, writing your reactions down helps you to remember the thoughts of an author.

–from How to Read a Book, by Mortimer Adler and Charles van Doren

Note: Do this only to books you own, never library books! Be considerate of the next reader.