Recently I have bought the book “Open-Source ESBs in Action” from Rademakers, Dirksen from Manning. It has a foreword from Guillaume Nodet, the famous Servicemix architect, which we know at blue elephant, so it has been a must-buy after it finally is the first book on Servicemix that there is. The book focuses on Mule and Servicemix, a pitty that OpenESB isn’t covered as a alternative approach, but well.
So far, the book is well-structured, providing a good insight also in side-topics and offers a lot of examples also with concrete configuration sniplets etc. I can recommend it.
The book web site http://www.esbinaction.com/ contains links, updates, articles and presentations on the topic.