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?

I'd hate to differentiate between the 'z' and 'Z' commands
or even worse, the 'O' and '0' commands or the 'I', 'l', and '1' commands.