If you're reading this, you probably don't need to be convinced about learning Spring. The question you're really asking yourself is, which book should I buy? Or should I just stick with the online docs and save some money? Or should I just download the code and start playing with it? I'm going to try to answer those questions.
First, Spring was born out of the thinking by Rod Johnson in "Expert One-on-One J2EE Design and Development", and later with this followup book co-authored with Juergen Hoeller in "Expert One-on-One J2EE Development without EJB". These books are excellent books in general and I highly recommend them. However, the first book is not really about Spring and is more about general enterprise application development strategies (and very good at that). The second is sort a essay on why EJB has failed, and also a short introduction to Spring along with the philosophy behind the design decision in Spring. However, it's too sparse to be a full-fledged Spring manual or tutorial. It's more a well-argued anti-EJB book with a short tour guide to Spring.
In contrast, Rob Harrop (who is also a Spring developer) has written the first truly comprehensive introduction and tutorial to Spring. It covers the whole gamut, from a clear introduction to why Dependency Injection makes a lot of sense, on Aspect Oriented Programming and how it applies to Spring, then on to a detailed coverage of how to use Spring for persistence, transactions, remoting, messaging, scheduling, email, and MVC web applications. He shows how to integrate with Hibernate, iBATIS, JDBC, JTA, JMS, Quarts, Struts, Velocity, etc.
What's amazing is that it covers Spring 1.2, which is still in release candidate stage, and gives you updates on the current stage of various supporting software, what to watch for in the near future and what the changes will most likely be. Just as one example, the Spring IDE plugin to Eclipse has really no online documentation to speak of since it's still relatively new, but this book shows you how to get it, install it, use it. The book has better documentation than the canonical website. That's just one example of many.
So how does this compare with the online docs for Spring? The online docs are good in most places, but there are still some big gaps in the documentation, whereas this book is nothing but exhaustive in its coverage and clearly superior in most places compared to the online-docs.
I've read both of Rod Johnson's books, the online docs and Rob Harrop's book, and this book is probably the best out there right now for its coverage of Spring, and it's also a one-stop shop. You don't really need anything else, unless you're looking to expand your knowledge with the other books into areas outside of Spring.
The only other book that might come close is the (as yet) unpublished book by Rod Johnson titled "Professional Java Development with the Spring Framework". However, that book is not out yet, so unless you can stall your Spring development for many months (hah, hah), I highly recommend Rob Harrop's book. Be productive and just get it.
