Charles Petzold is well known for high quality technical books and this is no cheap give away. It's a 1000 page book covering both Silverlight and XNA development.
The downloadable version is a PDF - about 39MBytes - and so should be readable on almost any machine or eBook reader. Although it has to be said that it hasn't been optimised for use on an ebook reader and the color markup on the code makes it difficult to read on a mono-chrome reader. The code is available as a separate zip file.
You can download the PDF from:
Programming Windows Phone 7
One can only assume that this is another effort by Microsoft to make sure that we get on with the job of creating apps for Phone 7. The book will be available as two paper editions later in the year - one on Silverlight and one on XNA.
Getting started with Windows Phone 7
Windows Phone 7 - Snakes on a Phone Part 1
Windows Phone 7 - using the Pivot control
Windows Phone 7 - using the Panorama control
Windows Phone 7 - the Bing Maps Control