I love writing in the books I am reading! I always have a pencil in hand when I read non-fiction. Recently, I read a book that has transformed my reading. It is How to Read a Book: The Classic Guide to Intelligent Reading by Mortimer J Adler and Charles Van Doren. This is a must read! Happily, they have confirmed my habit of writing in the books I read.

I would like to pass on these reading tips by Adler and Van Doren:

Here are three reasons to write in your book while you read:

"It keeps you awake."
"Reading, if it is active, is thinking, and thinking tends to express itself in words, spoken or written. The person who says he knows what he thinks but cannot express it usually does not know what he thinks."
"Writing your reactions down helps you to remember the thoughts of the author."

Here is how to do it:

“Marking a book is literally an expression of your differences or your agreements with the author.”

— Mortimer J Adler and Charles Van Doren

Underline major points and statements.
Vertical lines at the margins too long to be underlined.
Star, asterisk, or other doodad at the margin to mark important statements and passages (not too many). You could fold the corners of these pages for quick location.
Numbers in the margins of sequences and lists.
Numbers of other pages or sources that relate.
Circling of key words and phrases.
Writing in the margin and in the blank pages at front and back.