The PIL, http://www.lua.org/pil/, is as canonical as it gets, written by the creator of Lua.

Alternatively, there's also http://luatut.com/ for a quick useful intro

Really an awesome book! It not only teaches you about the programming language itself, but also how to use it well and idiomatically. Also, it is very focused and compact, which is a great thing in a programming book--although it makes for kind of a dry read.

This was posted here a while back: http://learnxinyminutes.com/docs/lua/

http://tylerneylon.com/a/learn-lua/ as simple as it can get. Lua is a very small (in term a amount of syntax) language.

I first started playing around with Lua here: http://repl.it/languages/Lua

Try http://learnxinyminutes.com/docs/lua/