Two books I enjoyed reading recently are:
Programming with POSIX Threads by David R. Butenhof (Addison-Wesley)
Very well written, concise and nicely layouted. Additionally to discussing the posix functions in detail, the second half of the book contains further material regarding worker thread pools, synchronization techniques, etc. Highly recommended.
Hacker's Delight by Henry S. Warren (Addison-Wesley, 2003)
Bit-twiddling to the max! This came in handy several times already, when needing to squeeze out some additional CPU cycles in innerloops.
See also: http://www.hackersdelight.org/
Check them out, you might also like them...