Structured approach to what? You don’t take a structured approach to a hammer, you use it as a tool to accomplish something.

”The Linux Programming Interface” is an excellent book, if you are interested in interacting with the Linux kernel directly, but somehow I doubt that’s what OP wants to do. I doubt OP knows what he wants to do.

Besides, please note that I did encourage taking a structured approach to stuff discovered on the way. But taking a structured approach to ”Linux” is just a bad idea, it’s far to broad of a topic.

Edit: RedHat has their certification programs. These are certainly structured. You’ll get to know RedHat and the RedHat^{TM} certified way of doing things. That’s probably the closest thing to what OP wants. You even get a paper at the end if you pay up. This is not the most efficient way to get proficient.