try these from Amazon.com
Hacking World of Warcraft (ExtremeTech) by Daniel Gilbert and James Whitehead II (Paperback - June 5, 2007)
World of Warcraft Programming: A Guide and Reference for Creating WoW Addons by James Whitehead II and Rick Roe (Paperback - Feb. 8, 2010)
Beginning Lua with World of Warcraft Add-ons by Paul Emmerich (Paperback - July 29, 2009)
or these free references
http://www.wowwiki.com/Lua
http://www.lua.org/
http://lua-users.org/wiki/SampleCode
hope this helps to get you started