I generally fallback on the O'Reilly [link|http://www.oreilly.com/catalog/dhtmlref2/|DHTML] and [link|http://www.oreilly.com/catalog/jscript4/|JavaScript] books. [link|http://www.dannyg.com/pubs/jsb/|Javascript Bible] is also good but after reading it, I rarely fall back to it for reference purposes. Anyhow, if you're in the market, those are probably the three best.

There's lots of web resources as well, but I usually just google and end up at random places. Two that I've bookmarked but don't use regularly are [link|http://javascript-reference.info/|Javascript crib] and [link|http://webref.info/default.asp?|WebRef].