These libraries provide widgets that seamlessly integrate with each other, delivering a consistent, high quality look and feel. In addition, all widgets are guaranteed to work on multiple devices and browsers ensuring maximum compatibility.
The authors describe thoroughly uncommon topics such as the ThemeRoller and the CSS Framework that allow developers and designers to extend the UI library looks and adapt it to existing styles within an existing application.
jQuery UI 1.10: The User Interface Library for jQuery is a great cookbook for Web application developers who are looking a go-to reference of the jQuery UI library. It doesn't make assumptions, its thorough and current.
If you are interested in understanding jQuery UI's library structure, or how each of its pieces work in depth and with great examples you should grab a copy.