People often recommend Douglas Crockford's "Javascript, the Good Parts" and "Eloquent Javascript," by Marijn Haverbeke. The source code of interesting libraries like jQuery can be very illuminating as well.
I recommend "Javascript: The Definitive Guide". It's not a "fun" read, but it will tell you almost everything you need to know. From javascript the language to the DOM and the different browser behaviour.<p>It also includes a reference section that is indispensable. I recommend getting it as an ebook though because you're going to be using the reference section a lot.
Here are some interesting web pages I've bookmarked on JS:<p><a href="http://www.bolinfest.com/javascript/inheritance.php" rel="nofollow">http://www.bolinfest.com/javascript/inheritance.php</a><p><a href="http://fuelyourcoding.com/jquery-events-stop-misusing-return-false/" rel="nofollow">http://fuelyourcoding.com/jquery-events-stop-misusing-return...</a><p>If you start reading them, and spider out and read more whenever you encounter something you don't understand, you'll learn a lot.
Do you need a book? If not, then <a href="http://developer.yahoo.com/yui/theater/" rel="nofollow">http://developer.yahoo.com/yui/theater/</a> Anything by Douglas Crockford is especially useful.<p>Warning: Douglas is only one man and he is very opinionated about some stuff but he is also smart. So, take the good and leave the bad. Or sometimes, you have to take the bad because everyone else in the world forces you to.
I've read "Murach's JavaScript and DOM Scripting" and I really liked it. It makes a solid foundation ...
<a href="http://www.murach.com/books/mdom/index.htm" rel="nofollow">http://www.murach.com/books/mdom/index.htm</a>