Deep book researching advice

Cal Newport divulges his technique for researching books.

The key to my system is the pencil mark in the page corner. This allows me later to quickly leaf through a book and immediately identify the small but crucial subset of pages that contain passages that relate to whatever project I happen to be working on.

How I Read When Researching a Book by Cal Newport

Such a simple thing to do and yet I wonder why I haven't read of this before. I often fold down the corner of pages, but simply putting a line in the top of the page I can quickly thumb through a book without having to worry about folded corners of pages unfolding themselves and being missed.