Great book for first principle introduction,
This review is from: Developer's Guide to Microsoft Prism 4: Building Modular MVVM Applications with Windows Presentation Foundation and Microsoft Silverlight (Patterns & Practices) (Paperback)
If you are looking for a first principles introduction to PRISM, MEF, Unity or MVVM and using them to write applications in WPF and Silverlight then this book is for you.
The overviews of each of the key topic areas are well written and easy to follow, as is the coverage of the design patterns that need to be understood.
I couldn't disagree more with some of the other reviewers, this is a very useful book and if you are moving into PRISM/MVVM reading it will help you significantly.