The Evolution of Lua
tecgraf.puc-rio.br
tecgraf.puc-rio.br
The first edition is free online, though there are some changes between Lua 5.0 and 5.1 (most notably: the module system and the syntax for variable-argument functions), so if you get into the language, it's worth buying PiL2.
In my experience, Lua is most similar to Python, but with a cleaner overall design, and a more pronounced Scheme influence (real lambdas, coroutines, lexical scoping, etc.). It's a "smaller" language, in much the same way that Scheme is smaller than Common Lisp. OTOH, they've made it very easy to use with C (both calling C libraries and as a scripting layer for C), so having a smaller standard library is not a big deal. It's also trivially portable. (It's supposed to be easily usable with C++, but I haven't tried.)
In particular, Javascript's JSON seems semantically identical to Lua's table notation (just with different braces in different places). Lua is used as a data serialization language as well (see the notes about Petrobras, for example), and the compiler has been tuned specifically for vacuuming up data files. It's also good as a ready-made config file format.
See: http://lua-users.org/wiki/LibrariesAndBindings, luaforge (http://luaforge.net/), and LuaRocks (http://luaforge.net/).