If you are looking for a book on SL4, then I can heartily recommend
Silverlight 4 Unleashed[
^] by Silverlight Ubermeister Laurent Bugnion. It's effectively two books in one, Silverlight 2 Unleashed to get you to grips with basic features and the second book goes into detail about what's new in Silverlight 4 and how to build high quality applications in Silverlight.