Atari Lynx Programming
AtariAge Atari Lynx programming forum
LX.NET's fantastic Atari Lynx programming tutorial
Listing of Atari Lynx programming tools, and tutorials
You should also understand the program flow.
Notepad++ is a nice advanced text editor for programming. (change syntax type to c/c++) and activate those line numbers so you can easily find your mistakes if the compilator complains. The function plug-in is cool too.