Wtb edit button. That should have said coding UIs and other addons.

Lua is free to use and requires nothing more than a text editor like notepad to get started. The easiest way to get started is to look at some simple addons that others have made. The are all plan text files and you can poke around in them all you want. http://www.wowpedia.org/Lua has links to more information on the language and advanced editors that can make working with it a little easier.

Lua is a scripting language thus there is no "buying" it. You can program Lua files with Emacs or notepad or what ever. The wow wiki has a list of editors better for it though. http://www.wowwiki.com/Lua_editors

WowWiki also has an enetire section of Lua resources. http://www.wowwiki.com/UI_FAQ/AddOn_Author_Resources

If you prefer hard copies there is a book called Beggining LUA with world of Warcraft Add-Ons. The publish bate is pre-cataclysm so many of the APIs have changed but the the programming instruction is still accurate. http://www.amazon.com/Beginning-Lua-World-Warcraft-Add-ons/dp/1430223715/ref=sr_1_1?ie=UTF8&qid=1302215752&sr=8-1-spell

Lua is a free and open-source language. The links here are a good place to start for info about Lua: http://www.wowpedia.org/Lua Aside from just understanding Lua by itself, you should also familiarize yourself with the WoW API. This is a great resource for that: http://wowprogramming.com/docs

Also, you might find it helpful to know that all addons run off of .lua files inside your addons directory. Just pull open any of your addons and you'll find tons of example code. If an addon is really simple it shouldn't be too hard to figure out what the code does especially if it's well commented (good luck with that, though).