The Linux kernel, along with much of the Linux system, is written in C. These two books rock that subject.
The C Programming Language . Prentice Hall, 1988. The definitive book on the C programming language, written by the author of C and his coworker. Peter van der Linden .
Expert C Programming . Prentice Hall, 1994. A great discussion on some of the less understood details in C. The author has a wonderful sense of humor and the book is rife with nonsense. I love it.