Reading DJB's code can be both awesome in that his code is written clearly and neatly, yet at the same time it can be an excercise in frustration because he has foregone most of the standard library and written his own (especially string management, which is superior, in my humble opinion), so it can look foreign or weird to people looking at it.
The source to qmail/daemontools is a pleasure to read though and having read it I feel like I have learned a lot from what it has to offer.
djb once wrote a FORTH implementation for the IOCC. It would be cool to get it running under current BSD/Linux/Solaris.
Arthur Whitney is another I would put on par with djb. He's a bit older than djb.
For expertise you can't beat W. Richard Stevens. He also studies and wrote about FORTH before focusing on solely on C.
Both Whitney and djb have a true appreciation for speed, efficiency and succinctness; both have solid foundations in maths; both can build very level abstractions. But they have different areas of focus.
djb - secure systems administration and networking.
(Stevens - documentation.)
Whitney - Lisp background; big data.
Whitney has proven that it's possible, using a matrix-based approach, to meet or beat the speed of C with an interpreted language.
But it's difficult to write UNIX systems or networking code without knowing C. For guidance on navigating the many pitfalls of C, djb and Stevens are as good as it gets.
The source to qmail/daemontools is a pleasure to read though and having read it I feel like I have learned a lot from what it has to offer.