I second JavaScript: The Good Parts[1]. It is very well-written and very dense, so you can read and reread it multiple times, each time gathering something new. Along with that I would recommend the Mozilla Developer Network JavaScript reference[2].

[1] http://www.amazon.com/JavaScript-Good-Parts-Douglas-Crockfor...

[2] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...