One of the ways to Do things with the steepest learning curve. More generally: ‣.

Look for places in the book that you don't understand. Aim to elevate your understanding only with the power of your mind alone (exception: philosophy).

Strive to read well to elevate yourself to the level of the authors.

The sentences important for you are those requiring an effort of interpretation or puzzling, not those interesting to you. The sentences important to you (stretching your understanding) are also likely to be important to the author because they are non-obvious.

Rule 6 of Analytical reading: Grasp the author's propositions by pondering key sentences.