"Stanford CS Education Library: A 45 page summary of the C language. The coverage is pretty quick, so it is most appropriate as review or for someone with some programming background in another language. Topics include variables, int types, floating point types, promotion, truncation, operators, control structures (if, while, for), functions, value parameters, reference parameters, structs, pointers, arrays, the preprocessor, and the standard C library functions."
The online version of The C Book, second edition, originally published by Addison Wesley in 1991.
A style-guide for developing and maintaining the Linux kernel. The advice in chapters 1-8 is generic and applies to any C programming project.