I picked up this book after reading a good review on Slashdot.org, and I'm happy to say that I'm not disappointed.
The book covers the installation of the server and the configuration (including a Jabber cluster), and then starts covering the XML protocol that is used to send information between servers and clients.
Several useful real-world examples are given, including a CVS-notification system, keyword assistant, headline viewer, etc. There is even a project to hook Jabber up to a coffee pot using Lego Mindstorm, with the point being to show how flexible Jabber can be. Examples are in PERL, Python and Java.
The book makes it quite clear that there is far more to Jabber than just instant-messaging.