I really can't attest to which language is the most widely used in the TC community. Certainly there's a lot out there documented in JScript, but I've seen people post questions about DelphiScript, VBScript, and others, so it's a pretty wide range of experiences. Of course, with Keyword Testing, scripting language is no longer really a HUGE concern (although using Code Snippets and Code Expressions require a little bit of knowledge).
While JavaScript and JScript are essentially two different things, the syntaxes are close enough, I think, that if you learn one, you should be able to use the other with minimal re-orientation required.
As for books and/or classes, that I'll leave to someone else to supply. For me, when I need resource on how to code something, that's when I crack open the "Google" search and see what I can find.