The RiscLua distribution always comes with at least a link to the reference manual for the relevant version of Lua. You will need to consult this for the details of Lua and its standard built-in libraries. See https://www.lua.org/docs.html for other Lua documentation. I also recommend Programming in Lua (4-th edition) by Roberto Ierusalimschy. Anybody interested in programming, in whatever language, will find much of interest in it. The final six chapters address the interface between Lua and C, which is analogous to the high-level versus low-level relationship between BASIC and ARM assembly language. These notes will look only at Lua and its extension to RiscLua, and not at the C aspects.

https://www.lua.org/ The official Lua website

https://lua-users.org/wiki/ The Lua Users website