A Microsoft disponibilizou vários livros, em versão digital, de forma gratuíta.
Resolvi compartilhar alguns links através deste post.
Seguem os links:

Own Your Space–Keep Yourself and Your Stuff Safe Online
http://www.microsoft.com/download/en/details.aspx?displayLang=en&id=1522

Windows Phone Programming in C# For Beginners
http://blogs.msdn.com/b/mvplead/archive/2011/01/28/free-ebook-windows-phone-programming-in-c-for-beginners.aspx

Silverlight for Windows Phone
http://blogs.msdn.com/b/lebanon/archive/2011/02/25/free-ebook-silverlight-for-windows-phone.aspx

Windows Phone Programming in C#
https://www.facultyresourcecenter.com/curriculum/pfv.aspx?ID=8729&c1=en-us&c2=0&Login=&wa=wsignin1.0

Programming Windows Phone 7
http://blogs.msdn.com/b/microsoft_press/archive/2010/10/28/free-ebook-programming-windows-phone-7-by-charles-petzold.aspx

Programming Windows Phone 7 (Special Excerpt 2)
http://blogs.msdn.com/b/microsoft_press/archive/2010/08/02/free-ebook-petzold-s-programming-windows-phone-7-special-excerpt-2.aspx

Own Your Future: Update Your Skills with Resources and Career Ideas from Microsoft
http://blogs.msdn.com/b/microsoft_press/archive/2010/03/03/free-ebook-own-your-future-update-your-skills-with-resources-and-career-ideas-from-microsoft.aspx

11 Steps to Create a Successful Web Site
http://blogs.msdn.com/b/officeliveguy/archive/2009/02/12/fre-ebook-11-steps-to-create-a-successful-web-site.aspx

Build a website that sells
http://blogs.msdn.com/b/officeliveguy/archive/2009/02/12/free-e-book-build-a-website-that-sells.aspx

Windows Azure
http://blogs.msdn.com/b/innov8showcase/archive/2010/07/06/free-e-book-on-windows-azure.aspx

First Look Microsoft Office 2010
http://blogs.msdn.com/b/microsoft_press/archive/2010/01/20/free-ebook-first-look-microsoft-office-2010.aspx

Microsoft Office 365: Connect and Collaborate Virtually Anywhere, Anytime
http://blogs.msdn.com/b/microsoft_press/archive/2011/08/17/free-ebook-microsoft-office-365-connect-and-collaborate-virtually-anywhere-anytime.aspx

Moving to Microsoft Visual Studio 2010
http://blogs.msdn.com/b/microsoft_press/archive/2010/09/13/free-ebook-moving-to-microsoft-visual-studio-2010.aspx

Windows 7 troubleshooting tips
http://blogs.msdn.com/b/microsoft_press/archive/2009/10/26/free-e-book-windows-7-troubleshooting-tips.aspx

Understanding Microsoft Virtualization Solutions
http://blogs.msdn.com/b/microsoft_press/archive/2010/02/16/free-ebook-understanding-microsoft-virtualization-r2-solutions.aspx

Introducing Microsoft SQL Server 2008 R2
http://blogs.msdn.com/b/microsoft_press/archive/2010/04/14/free-ebook-introducing-microsoft-sql-server-2008-r2.aspx

Mais livros em http://social.technet.microsoft.com/search/en-US?query=Free%20e-book%3a&startindex=0

Por
Fernando Henrique Inocêncio Borba Ferreira.