When reading during study I recommend marking important points in the margins with a pencil tick or highlighting keywords. Adding margin notes or marginalia while reading is called ‘active reading’. It keeps you more mentally alert.
(Some students even seek out second hand textbooks with marginalia added as a guide to own study. So don’t worry about resale value – add notes.)
There are many ways to mark up text beyond simple ticks or highlighting. Develop your own repertoire of symbols or personal codes to add in the margin e.g. three asterisks if truly important, question mark or exclamation, arrows to indicate relationships.
For a good discussion on how to mark a book while reading, read Mortimer J. Adler’s classic, ‘How To Read A Book: The Classic Guide to Intelligent Reading.’ Blogger Cris Tovani, also a fan of Mortimer Adler’s essay ‘How To Mark A Book’, cites another benefit to adding marginalia or annotating, ‘When I annotate, I leave tracks of my thinking on the page. Later, I can read those annotations and be reminded of what was happening in my head at that specific moment.’
So adding marginalia gives us not only better memory but self knowledge over time.
I’m interested to observe how adding electonic marginalia is evolving. We can highlight words and add comments and electronic sticky notes to PDF documents. A trend is to be able to mark up documents electronically with digital ink.