My number one complaint about University Courses. We didn't spend a single minute doing what most of my job consists of: reading someone elses code and trying to decipher what the heck they were trying to do.

Like reading a human language, once you can "sound it out", so to speak, it's something once typically gets better at by practicing.

With well-structured code, as with a well-structured essay, you can learn to skim to get an overview before your deep dive by focusing on the beginning and end of every chapter/paragraph/etc (e.g. function definition and return statement; gate for each block; treating header files as Table of Contents; etc). But most code is no better structured than your average Tumblr post, so that's kind of hit and miss.

And as with reading human language, you can learn techniques for taking notes as you go along to guide yourself or remember important details; but the technique that one person finds helpful may be a waste of time for another.

A book won’t teach you, pick a large open source project you use daily like nginx or apache and study it starting at main.