That book is a little dated because it deals with Lua 5.0 but is still good. The differences between 5.0 and 5.1 can be found here http://www.lua.org/manual/5.1/manual.html#7 and new version of the book can be found on amazon.

It will still give you a good taste of the language, but PiL 2nd ed. and the reference manual will be much more useful in the long run.

Also available on its official site: http://www.lua.org/pil/