Honestly, I think Lua and Python are the most recommended languages to learn for scripting. As I've stated many times during my advice, there is no such thing as a "waste of time" when learning a language. Every language you learn gives a different perspective of how to solve problems as every language has different ways of solving them. The only time something becomes a waste is when you get tired of doing it. There is always something fun about learning a new language.

It depends. Do you intend on becoming a programmer or designer?

Lua may be useful to a programmer, though it is arguably just "another script" and can be learned on-the-side when needed.

As a designer though, there are quite a few more "technical" positions you can open up for yourself when you know at least one scripting language. In my area, AAA studios rarely hire for junior designers, but they occasionally hire designers with a technical background as it avoids them the need to train in-house and has value for them.

Make yourself valuable = get yourself hired!

maybe get a textbook

Not for Lua. It really is trivial.

Eehhh I actually liked the Programming in Lua book and have it here with me now (the orange version). Some of the concepts are just sort of weird, like metatables and whatnot, so I like to have a language reference. But, the Lua website has pretty much all of this info so the book isn't really necessary.

You can just read the online version instead of forking out before knowing whether you need it

http://www.lua.org/pil/contents.html

This is for Lua 5.0. Latest is 5.3.2 just making that known so there will be significant differences.

Lua was my first programming language then I transitioned to Java and C#. One of my C# Projects will integrate Lua. Its pretty easy to learn, simple, fast, extendible. Go for it.

...I know C++ and C#