Why build Windows 8 apps? When you create a Windows 8 app, you can put your app in the Windows 8 Store. In other words, customers can buy your app directly from Windows. Think iPhone apps, but for a much larger market.
You can buy the book by going to your local Barnes & Noble bookstore or you can buy the book through Amazon by using the following link:
It looks like the book is also available for the Kindle: