You mentioned that you have used samples in your book when explaining the different features/usages of the ESB. Are you using specific implementetion of ESB to do this? And is there a way we can do something in our own to test the ESB funtionality after reading the book? Do you have done some samples with real coding?
The book focuses on Mule and ServiceMix and we've made all examples available for everyone via Google code. You can use the links on the book's website (http://www.manning.com/rademakers) to go the source code. We've also provided a setup script that installs a complete environment which you can also use to build your own projects.
Author of Activiti in Action and Open Source ESBs in Action
We didn't do a feature for feature comparison of Mule and ServiceMix in the book. We do however implement all the examples for both these ESBs. So with regards to functionality offered they are pretty much equal.
There are a couple of differences though between these ESBs; For instance ServiceMix is much more XML focussed and has a more extensive deployment mechanisms and Mule is more lightweight and flexible
Both though are very mature ESBs with lots of functionality.