You should always know what to highlight. I saw so many people highlighting just for fun and to make pretty notes. Which you can also do, but highlighting text you're studying is q key for easier learning. It is truth that some texts in books are good and important so it happens to highlight whole text. But I can't imagine myself learning without highlighting. I have feeling that I can't see the text, like it's unimportant. 1. Always read every sentence in the book (and also those additional information) and highlight each one you can make a story from and understand fast.