Useful advice when reading a book ...

... but make sure it's your own book: 'How to mark a book' by Mortimer J. Adler:

There are all kinds of devices for marking a book intelligently and fruitfully. Here's the way I do it:

• Underlining (or highlighting): of major points, of important or forceful statements.
• Vertical lines at the margin: to emphasize a statement already underlined.
• Star, asterisk, or other doo-dad at the margin: to be used sparingly, to emphasize the ten or twenty most important statements in the book. (You may want to fold the bottom comer of each page on which you use such marks. It won't hurt the sturdy paper on which most modern books are printed, and you will be able take the book off the shelf at any time and, by opening it at the folded-corner page, refresh your recollection of the book.)
• Numbers in the margin: to indicate the sequence of points the author makes in developing a single argument.
• Numbers of other pages in the margin: to indicate where else in the book the author made points relevant to the point marked; to tie up the ideas in a book, which, though they may be separated by many pages, belong together.
• Circling or highlighting of key words or phrases.
• Writing in the margin, or at the top or bottom of the page, for the sake of: recording questions (and perhaps answers) which a passage raised in your mind; reducing a complicated discussion to a simple statement; recording the sequence of major points right through the books. I use the end-papers at the back of the book to make a personal index of the author's points in the order of their appearance.

The front end-papers are to me the most important. Some people reserve them for a fancy bookplate. I reserve them for fancy thinking. After I have finished reading the book and making my personal index on the back end-papers, I turn to the front and try to outline the book, not page by page or point by point (I've already done that at the back), but as an integrated structure, with a basic unity and an order of parts. This outline is, to me, the measure of my understanding of the work.

I tend to use lines in the margin to mark something important - the more marks the more interesting/ important I feel it is. I also use an exclamation mark if it is some thing surprising and a question mark if I'm not sure about what is written. I sometimes put an x if I think it is wrong.