The book does not cover Dojo specifically. Grails supports AJAX calls through a number of tags that abstract the underlying AJAX framework. By default the Prototype framework is used. There is a plug-in for Dojo, which is as simple to install as:
Then you tell Grails to use Dojo library in the head of your GSP file:
In the book I show how to use the Grails AJAX tags, and then go on to use the RichUI plug-in, which provides a number of common UI design patterns. The two I show are:
I'm currently working with Dojo and Grails. Unfortunately I cannot recommend installing the dojo plugin. It's totally outdated. Regarding to the Grails mailing list the dojo plugin will be updated to version 1.3 but until that the best way is to get the latest version (1.3) from the offical website and unpack it to your web-app/js directory.
In my opinion it's a great experience using Dojo and Grails together. Compared to YUI dojo gives you a lot more widgets. Only Unit-Testing is unbeatable in YUI. ExtJS is a great framework but through their license problems I cannot recommend it any more.