Enterprise Integration Patterns
I've been reading an excellent book lately: Enterprise Integration Patterns, by Gregor Hohpe, Addison-Wesley. ISBN: 0321200683 (Martin Fowler's Signature Series)
If you are interested in learning more about message-based enterprise integratioin architecture, this is a great resource. It is well written and very comprehensive. It gives examples for both the J2EE and the .NET crowds.
The only drawback to the book is that it only discusses message-based solutions. Still, that covers about 80% of integrations scenarios.
If you are a fan of patterns, you might as well check out the companion book in the series: Patterns of Enterprise Application Architecture, Martin Fowler, Addison-Wesley. ISBN: 0321127420
Enjoy!