None the less, I thought this would be an excellent occasion to do a free e-book round-up for Lua. Whether you want to learn it for game scripting, extending Renoise or building Web servers, these references will help you get started with the language.
The Lua community makes finding resources for leaning Lua quite easy. The Lua-Users wiki has a page dedicated to learning the language.
There's also a Web-based demo where you can enter your code and experiment with it.
Programming in Lua
The first edition of Programming in Lua by Roberto Ierusalimschy is available for free online. Ierusalimschy is the chief architect of Lua. The book was written with Lua 5.0 in mind (it's up to 5.1 now, and the 5.2 alpha is available for testing), but is still a good beginner's reference.
You can buy the second edition here.
Lua Programming Gems
Lua Programming Gems edited by L. H. de Figueiredo, W. Celes, R. Ierusalimschy introduces more advanced Lua concepts. Parts of it are available for free in online. The full book can be bought in print and various digital formats.
Lua Functional Programming
Lua Functional Programming is a Wikibook in progres "inspired by and based on Paul Graham's work On Lisp." It's meant for programmers already experienced with Lua.
Lua 5.1 Reference Manual
For those that just want to dive into the reference manual, the whole thing is available online for free.