It's ofcourse, about low level programming, but since, the lower levels underpin the higher ones, for me the most useful thing was this book.
It was a super challenge for me personally (no degree in computing or anything), and working through it i felt more things click than any other resource i tapped into before. I feel i understand much better in any language now, what actually happens on the PC, and how to go about investigating what happens if i do not. (finding specifications, isa documents, documentation on interpreters of scripts etc. etc., learning to read code/sources.)
That being said, do you have a specific direction in which you want to improve? Because this obviously leans towards a specific flavor of programming, where it's maybe not as useful if you want a different flavor. I know many good/professional programmers who know nothing of this stuff and are really really good in their own domains.