If you are interested in asynchronous programming in .NET, Async in C# 5.0 does a really nice job of teaching you how to leverage the new async features in C# 5. I recommend the book not only for learning the new features in C# 5, but for also understanding various patterns and solutions used in the past and how the new features make it easier.
I really like the approach used in Async in C# 5.0 to teach the material. This isn't an easy topic by any means, and I think the author did a really nice job of teaching the material. Here is his approach and why I enjoyed the book so much.
The book is small and only took me 2-3 hours to read. The book flows well and is enjoyable to read, which means you will probably read it front-to-back in one sitting. Once you finish the book I dare you not to jump on your development machine. I immediately grabbed a cup of tea and started playing with aync programming in ASP.NET MVC 4 and Orchard CMS after testing the waters in a console application :)
My only disappointment with the book is that it focused on the same overused WebClient Sample. In this case it was doing several favicon downloads asychronously in a windows app. It's a good example, but a little overused and I would have liked to see more examples in the book and more in the downloadable sample code. The book mentions how most .NET developers are web developers but doesn't give a good ASP.NET MVC Example. There are a total of 2.5 pages focused on ASP.NET MVC and WebForms and I think several examples in ASP.NET would have been appropriate. At least add it to the sample code :)
Overall, I really enjoyed the book. I liked the writing style and the good information. It loses a star only because there should have been more and better examples both discussed in the book and provided in the sample code. I received this book for free as part of the O'Reilly Blogger Review Program, which is where I get most of my books these days :)
Learn more about Async in C# 5.0 on the O'Reilly Website.