Lua

The language of ToME4.

Here are some helpful links to learn Lua:

http://luatut.com/crash_course.html

http://lua-users.org/wiki/LuaDirectory

http://www.lua.org/manual/5.1/manual.html

http://www.lua.org/pil/ (Outdated but still useful)

http://lua.gts-stolberg.de/en/index.php (possibly outdated but this is an amazing resource, especially for beginners, I wish I would have known about it two years ago)

Once you know the basics of Lua you can start looking at the code of ToME4 and try to understand it, making small edits and observing if they do what you expect them to do to further increase your understanding.

Some comments on the talent code can also be found on this page.