[gebe]

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

[oblique63]

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

[Derbasti]

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.