Here are some guides I have found:
http://www.gnu.org/manual/glibc-2.2.3/ - the best one - complete desc. of linux's C standard library
http://www.ulib.org/webRoot/Books/Numerical_Recipes/ - a book with numerical codes, also in C (F, F90 and C++ as well)
http://www.infosys.utas.edu.au/info/documentation/C/CStdLib.html - ANSI standard library - HARDCORE!!! - but necesarry
There were some useful guides but now seem abandomed - perhaps I have them at my home comp.
You can also do a 'info gcc' to have a description of the compiler and the language - quite useful! You might also want to search with www.google.com - sure there are lot of c-guides.
regards - Michal