If you're a programmer, Microsoft Press has released a free eBook that may be of interest -- especially if you have interest in developing for mobile platforms. You can download the eBook Programming Windows Phone 7, authored by Charles Petzold at no charge. It's hefty at almost 1000 pages, and the PDF file is 38.6MB. The book will teach you the basics of writing applications for Windows Phone 7 using C# with the Silverlight and XNA 2D frameworks.

On the page linked above, you'll also find some related resources including including links to the free online book .NET Book Zero: What the C or C++ Programmer Needs to Know about C# and the .NET Framework and Windows Phone Developer Tools, among others.