Mad C Skillz OEMG

I want to jump to the next level in C. I'm a quite skilled programmer for what I do, know the whole K&R but I feel like I miss something, especially about those weird techniques proPROgrammers use in OS dev and related. Suggested readings?

Here's what I don't understand: basic Unix commands are largely two-letter abbreviations. Why didn't they just make them one letter? Most of the two-letter commands could simply have their second letter removed with no other changes necessary, and be typed twice as quickly and easily.