Not all the following resources are not required material, but may help to get you started or keep you rolling:

Course material linked elsewhere but included here for your reference

• Hanson Quick Reference
• Course coding standards
• C Idioms useful for CS 40
• Pair programming FAQ
• Pair programming code of conduct

Transitioning from C++ to C

• Mark Sheldon's whirlwind tour of C
• C idioms for CS 40. These will definitely come in handy.
• Andrew Koenig's C Traps and Pitfalls
• Dinkumware's documentation for the Standard C Library

Compiling programs, building and debugging applications

• An Introduction to Compile scripts and Makefiles
• Compiling quickly using emacs or vim
• How to Write Compile Scripts
• What happens when you build (compile/link) a C program?
• Understanding valgrind output

Other textbooks and articles

• The C Programming Language by Kernighan and Ritchie
• The UNIX Programming Environment by Kernighan and Pike
• The Pracitce of Programming by Kernighan and Pike
• The Elements of Programming Style by Brian W. Kernighan
• Expert C Programming: Deep C Secrets by Peter van der Linden
• Extreme Programming Explained by Kent Beck
• What Every Computer Scientist Should Know About Floating-Point Arithmetic by David Goldberg
• Literate programming simplified by Norman Ramsey
• GNU Make by Richard M. Stallman, Roland McGrath, and Paul D. Smith