Hi Jeff,
In general, I've heard good things about codecademy's javascript course:
I've been recommending that.
Another resource you can look at is w3schools:
I tend to go to that website if I'm looking up the API for various things in JS:
(For example, it's easy to forget the exact method to use for
dates or
strings).
If you're looking for actual books, O'Reilly and "For Dummies" tend to be "go-to"s for learning programming languages. That said, I don't think you should require more than a basic understanding of javascript. I think the books will probably cover a lot more than is required for AdWords scripts. The codecademy course goes over functions, for/while loops, if/else conditions, objects, and methods. I think that provides a strong enough base of JS knowledge for AdWords scripts. There's a lot more stuff in JS that you would want to learn if you were using it to build a website (how to update the DOM, how to handle events, how to set up alerts, etc), but are completely unnecessary for AdWords scripts. My recommendation is to try an online course first and if you're still interested in learning more javascript, then look for a book.
Cheers,
Alex