Microsoft has released a Windows 8 Developer Preview Guide and it is now available for download.
This 21 page guide will be very useful for Windows 8 enthusiasts in general and Windows 8 developers in special. Not only does it cover the main features of Windows 8, but also gives useful tips to app developers.
When you create a Metro style app, you get deep integration with the new Windows 8 features, including the app bar, edge, live tiles, and contracts with other apps. Your app participates fully in the touch-first interface and modern user experience. When it runs, your app becomes the focus of attention, and the operating system chrome recedes into the background.
You can download PDF from here.
Windows 8 fact sheet may also interest you!