If you are looking to build a business-focused application with Silverlight, chances are you are going to need to access data of some sort. This is the best book I have found to address this aspect of Silverlight development.
Virtually every method of data access currently available in Silverlight is covered here: LINQ, REST, WCF and more.
I read the book cover-to-cover, but I think it will serve best as a reference guide while developing line-of-business apps with Silverlight. Most of what you see written about Silverlight focuses on shiny UIs. This book gives you what you really need to build RIAs for real business requirements.