Buy a copy of
World of Warcraft Programming, Second Edition. Written for coding during Wrath of the Lich King, the companion website
http://www.wowprogramming.com is current. All the basic fundamentals of learning WoW's Lua is in the book, and all that stuff is still very valid.
For updated references to WoW's API, see both the link above and
http://wow.gamepedia.com/Portal:Interface_customization