Here's a great site from Penn State.
Sometimes it's called "annotating"
Here's my short take on it:
Why do it?
1. Don't read in bed.
and don't read with TV/Music
2. Don't use a highlighter in the book.
3. Look for things going in.
4. Make assumptions.
5. Ask questions.
6. In N&D, the footnotes are full of gold.
Here's what my book looks like.
6. At outline stage, go back through and make a list of pages and quotes.