Marking the essentials (or better essentials based on your your current understanding) is different from marking whole sections or pages. I remember one of my

Marking the essentials (or better essentials based on your your current understanding) is different from marking whole sections or pages. I remember one of my teachers saying he needs some books like Goethes Faust about every 10 years because of new understanding makes him mark different passages (in a different color) than on a previous reading. And after teaching and using the books for about 10 years he needs a fresh page to mark.