The reference http://www.arduino.cc/en/Reference/HomePage
very good summary of the C language http://cslibrary.stanford.edu/101/EssentialC.pdf
Reference for the standard C API (libc): http://www.nongnu.org/avr-libc/user-manual/modules.html To use with Arduino, add the appropriate #include to the top of your sketch and use the functions as documented in the manual.
Learn how to decipher all those cryptic formulas you see in C code that use symbols like
<<. Start using bitwise operators in your own code to save memory and perform sophisticated low-level operations on your microcontroller hardware.
More explanation of ports and manipulating them with bitwise operators.