As mentioned in http://www.winsupersite.com/blog/supersite-blog-39/windows8/…
'From June 1, 2012 until sometime in July, the cost will be $20. And then $30, and the eventually the book will reach its final and normal cost of $50.
'The catch is that this is a book in progress. So the version you’re getting today is the so-called Consumer Preview version, and it covers 7 chapters over 294 pages (described below), focusing on C# and XAML technologies. Mr. Petzold will be updating the book over time, with a Release Preview version expected sometime in July and then the final version later in the year. Anyone who buys the book qualifies for free upgrades to future versions. So if you buy today, you’ll get the subsequent revisions as they are published.'
As mentioned by supabrudda in the comments, use the coupon code ERR20 to get it for $8.
Wow this is so unprofessional, no way I would go for a book like this