Book publishing as an arm of marketing has a long history and usually the end user is the winner. In this case we have some free programming ebooks from Microsoft for you to download.

Microsoft really does want Windows 8 to be a success so it has a arranged for a free ebook on the subject of app development:

This is a full 300 page book, and not a preview or extract edition.

"This book is about writing Windows Store apps using HTML5, CSS3, and JavaScript. Our primary focus will be on applying these web technologies within the Windows 8 platform, where there are unique considerations, and not on exploring the details of those web technologies themselves. For the most part, then, I'm assuming that you're already at least somewhat conversant with these standards. We will cover some of the more salient areas like the CSS grid, which is central to app layout, but otherwise I trust that you're capable of finding appropriate references for most everything else."

Currently it is only available as a PDF but the website is promising EPUB and MOBI formats.

Slightly less useful as a free book, given the release of WP8, is:

Programming Windows Phone 7, by Charles Petzold

Similarly you might like

Moving to Microsoft Visual Studio 2010

but it is more likely that you are considering the move to 2012.

Let's hope that Microsoft make the later versions of both these books available in the near future.

Metro Revealed: Building Windows 8 apps with HTML5 and JavaScript

Metro Revealed: Building Windows 8 apps with XAML and C#

WinRT Revealed