Ebook: Open-Source ESBs in Action: Example Implementations in Mule and ServiceMix

A good book. + I got the pdf version free.. So in case you are planning to buy only PDF version, you can put few bucks extra and get hard copy and soft copy too. I particularly wanted this for Mule.
*** However for mule I feel the mule source documentation and user guide (on mulesource.org) is also quite well (if u have read EAI book by Harpe and Woolf).. especially if you try out the examples on your own.