Good Guides
http://lua-users.org/wiki/TutorialDirectory
http://www.lua.org/pil
http://wowprogramming.com/
API References
http://wowprogramming.com/docs
http://www.wowwiki.com/Api
Weird stuff but sometimes useful (although most guides in there are out of date)
http://www.wowwiki.com/HOWTOs
If all else fails and your really stuck
Make a thread here or go to the WoWUIDev IRC Channel.