Dojo is an open source effort managed by the Dojo Foundation. Unlike the other libraries, which are maintained by individuals or small groups, the Dojo effort actively encourages additions to the Ajax library from various sources.
The basic Dojo Toolkit is actually quite large and provides a rather complex "pluggable" framework, which allows for the development of Dojo-style widgets—external extensions that don't impact the underlying code. You can download the toolkit at http://dojotoolkit.org.
Dojo has extensive documentation, accessible at http://manual.dojotoolkit.org/index.html. The mechanics of widget-building for Dojo are beyond this book, and I'll refer you to a tutorial on the subject at http://openrico.org/demos/weather_demo.
Like other Ajax libraries, Dojo originally had little documentation. Considering how complex it was, this was more than a small handicap in using the toolkit. Luckily, the project did start a wiki (a site where more than one person can edit the contents) for documentation, though like all wikis, the coverage can be sketchy. Another place to find documentation is at ...