Rick Strahl and Matt Berseth's blogs both tipped me into jQuery and man am I glad they did. jQuery completely changes a) your client programming perspective, b) the grief it causes it you, and c) how much fun it can be!
http://www.west-wind.com/weblog/
http://mattberseth.com/
I used the book jQuery in Action
http://www.amazon.com/jQuery-Action-Bear-Bibeault/dp/1933988355/ref=sr_1_1?ie=UTF8&s=books&qid=1219716122&sr=1-1 (I bought it used at Amazon for about $22). It has been a big help into bootstrapping me into jQuery. The documentation at jquery.com are also very helpful.
A place where jQuery falls a little flat is with its UI components. Those don't seem to be quite ready for primetime just yet.
It could be that Prototype or MooTools or ExtJS are as good as jQuery. But for me, jQuery seems to have a little more momentum behind it right now and that counts for something for me.
Check jQuery out. It is very cool!