Lua Programming Gems was released around christmas and I think it's pretty good (although you definitely want PiL first). The mailing list is very friendly and there's plenty of information available on the lua-users.org wiki. Also take a look at Penlight (http://penlight.luaforge.net/), which tries to add a collection of useful helper functions to Lua. I haven't explored it in detail yet, but it looks like the strongest attempt so far. Finally, The Implementation of Lua 5.0 (http://www.tecgraf.puc-rio.br/~lhf/ftp/doc/jucs05.pdf) is well-worth reading. It has some interesting discussion about the move from a stack-based (pre-5.0) to a register-based virtual machine.