The notes by my OS lab professor, written in Romanian, with self-learning in mind, with detailed explanations of the workings of major system calls and featuring numerous working examples and pitfalls.
I might translate them some day and put them up on the Internet (if I get his permission and some free time).
Love - Linux System Programming
The Unix-Haters Handbook
(Unfortunately, the best resource ever for this kind of stuff, from which I learned, does not have an English translation.)