Also good: "The Implementation of Lua 5.0" (http://www.tecgraf.puc-rio.br/~lhf/ftp/doc/sblp2005.pdf).

Past discussion: http://news.ycombinator.com/item?id=617278

If you want to learn Lua, Ierusalimschy's _Programming in Lua_ (http://www.inf.puc-rio.br/~roberto/pil2/) is by far the best book. Lua is one of my favorite languages. While it's good on its own, you'll get a lot more out of it if you're comfortable with C. Lua+C is an excellent combination.