Just find the function that does what you want to do in the docsI often go looking for the wrong thing. I'll think "In Ruby/plain JavaScript/C/whatever, I'd achieve X by doing Y and then Z" and then look for jQuery's equivalents to Y and Z, rather than doing it the easier and conventional jQuery way.
Books are good at relaying the thought processes of experts rather than the function by function material that, yes, a reference site provides a lot better.
For a slightly convoluted example, let's say I have a little JavaScript and DOM knowledge and am a few days into using jQuery without following any serious tutorials or books. To change the code within an element I might try something like $('#whatever').innerHTML = 'whatever' rather than the more jQueryesque $('whatever').html('whatever') - a simple example but it can get a lot more elaborate than that.
However, it's definitely a case of different strokes for different folks. You might be able to pick up on the idioms and nuances of new libraries and languages without much guidance, but books like Rebecca's can provide a much needed "guided tour" for some people nonetheless.
All of the above is why I wrote Beginning Ruby despite the existence of the venerable Pickaxe. The Pickaxe is an awesome reference book with a relatively weak tutorialesque set of chapters tacked on the front. My book caters to people who want to learn in a more linear, narrative fashion (and, to be fair, has a relatively weak reference section tacked on the end ;-))