Two questions here:
1. What audience
2. When to use SI
ad 1:
Any technical person that is considering to use Spring Integration would benefit from the book. Also if you're using it already there are some pointers in there that might save you some time, or open your eyes to something. We've really tried to make it worth the read, so I hope you enjoy it.
ad 2:
This is a more tricky question. I would say that if you're processing a fair bit of information and you're doing several different things with it functionally
you should consider using Spring Integration for its runtime decoupling of services. This will help if you want to distribute the application over multiple functionally distinct nodes later. If you have a straightforward web-app or a straight forward batch job or a straight forward map-reduce job you might want to look at something else. If it's a little bit of two or more of those, Spring Integration might help you.
Another reason to use Spring Integration might be if you want your application to react to events (other than http requests). Coding that yourself is error prone and the abstractions that Spring Integration has for that are rather nice (in my biased opinion).
