Do I pick up any beginners book on Lua?
There really is only one beginners book on Lua :-) _Programming in Lua_, by Roberto Ierusalimschy, the Lua author. There are a few other books, mostly about "Lua and games", because Lua has become quite popular in the gamers' community, but Roberto's book is really excellent and makes a good tutorial.
For example, one of the lines in the code is
tprint = function(s) tex.sprint(tex.ctxcatcodes,s) end
Is this Lua or LuaTex?
The question does not really makes sense if you put it that way. This line of code is pure Lua, it is completely syntactically correct; but you can't use as it a standalone Lua script, because it uses functions that only LuaTeX and ConTeXt define (think of it as the action of "import" in Python, except that here it's not available as a stand-alone library call; it's built into LuaTeX). Hence, if you want to become proficient in Lua programming in ConTeXt, you need to learn: * The Lua programming language, with its core libraries (covered in the aforementioned book). * The Lua libraries that LuaTeX adds to Lua, documented in the LuaTeX manual. * The Lua libraries that ConTeXt adds to LuaTeX. The above line of code is really a mixture of the three, but in any case, the ground stone is always Lua. Arthur