Lua has been selected as the scripting language of choice because of its speed, compactness, ease of embedding and most of all its gentle learning curve. These characteristics allow the user to create simple scripts right through to advanced programming solutions that a computing science graduate would relish. In other words, Lua has all the depth and sophistication of a modern language, yet remains very accessible to the non-programmer.
Roberto Ierusalimschy’s ‘Lua 5.3 Reference Manual’ and ‘Programming in Lua, 3rd Edition’ and more recently “Lua Programming Gems” edited by Luiz Henrique de Figueiredo et al. These authors are also the authors of the Lua language. The books are highly recommended and there purchase helps support the Lua Project.
The official Lua site: www.lua.org a great resource covering most aspects of Lua, with many links.
Lua users www.lua-users.org lots of stuff
Tutorial for beginners www.tutorialspoint.com
Wikipedia www.wikipedia.org useful general information.
Wikibooks www.wikibooks.org not up to date but none the less useful
A crash course www.luatut.com useful.
A brief summary : LuaShortRef53.pdf that loads in you default PDF reader.
A description of FlyingSticks' Lua Extensions.