For more information on the Lua programming language I recommend the online Programming in Lua book. That is the best one to start with since it introduces Lua concepts in more of a tutorial manner.
Some sections to skip in the book would be the information on co-routines. Meta tables are probably not a useful concept unless you are going to get heavily into writing library code. I did find a use for it in the codemap example which ships with Iguana.
After you have learned the language then the Lua 5.1 Reference Manual is useful. Especially the sections covering the standard libraries.
Our technical writer found the Lua Tutorial Directory a useful resource. I have not delved much into this although I am told it is good.