Since the first jQuery homepage in 2006, an interactive example introduced visitors to jQuery with a single line of code, and a button to run that code. When clicked, it added a class to a hidden paragraph, and animated that paragraph to become visible.
Today, in late 2010, the API documentation has 15 methods listed in the Effects category. These provide built-in animations for fading and sliding, as well as various ways to create custom animations. When combined with color and class animations and custom easings that jQuery UI provides, there are even more ways to create animations.
A good rule of thumb for using animations is to use slides when showing elements within the pageflow, and fades for overlays, like a tooltip. But that's just a rule of thumb, and with all the tools available there's a lot more opportunity to improve interactions, as well as messing them up.
With that in mind, a full book on animations starts to make a lot of sense. It makes even more sense when also taking into account upcoming technologies which aren't bound to jQuery directly, like CSS3 animations or animated canvas drawings.
As a tech reviewer I've worked with Dan on his jQuery UI 1.6 and jQuery UI 1.7 books. At the time the jQuery UI team was still figuring out the scope and exact direction of the project, including several direction changes at the time when Dan was writing the first book. Despite these challenges Dan did a great job providing documentation and extensive examples on how to use and combine the widgets and interactions jQuery UI provides.
With this book Dan brings his experience in writing on jQuery topics to teach you when and how to use animations to create better user experiences. I hope it serves you well.