In the light of Microsoft’s decision on the future of Silverlight it doesn't sound heroic that I read "Silverlight 4 in Action" also taking into account that Pete Brown already released new edition of book for latest and last 5-th version of SL.
But I had read book couple of months ago before SL5 was released. I read it with my friends at work and it was joy.
It is somewhat standard step by step tutorial on particular technology. But it provides really comprehensive look at Silverlight.
If you use this technology, book is great asset to understanding Silverlight.