Arduino API

The reference http://www.arduino.cc/en/Reference/HomePage

C Language

very good summary of the C language http://cslibrary.stanford.edu/101/EssentialC.pdf

AVR Lib C

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.

Bit Math Tutorial

Learn how to decipher all those cryptic formulas you see in C code that use symbols like &= and <<. Start using bitwise operators in your own code to save memory and perform sophisticated low-level operations on your microcontroller hardware. https://playground.arduino.cc/Code/BitMath

Port Manipulation

More explanation of ports and manipulating them with bitwise operators.
Port Manipulation