Hands down *THE* book to buy...
http://wowprogramming.com/
Two tools worthy of use for WoW development:
http://www.wowinterface.com/downloads/cat49.html or my personal preference
http://www.google.com/url?sa=t&sourc...yz904LEi4nnZxA (nUI includes "solutions" for use with AddOn Studio in the distribution)
For quick reference, everything you need to know (almost) is here:
http://www.wowwiki.com/Portal:Interface_customization
After that, it all comes down to just your personal creativity and programming experience.