Found a page on the Technet site that provides a pretty good list of eBooks on various Microsoft Technologies

http://social.technet.microsoft.com/wiki/contents/articles/11608.e-book-gallery-for-microsoft-technologies.aspx

Topics covered

Office, SharePoint, SQL Server, System Center, Visual Studio, Web Development, Windows, Windows Azure, Windows Phone and Windows Server

Here is a twitter page also that free Microsoft eBooks get posted to as well

https://twitter.com/MSFreeEBooks