Microsoft’s Eric Ligman has made more than 130 technical books free to download. Topics include: Windows 8.1, Windows 8, Windows 7, Office 2013, Office 365, Office 2010, SharePoint 2013, Dynamics CRM, PowerShell, Exchange Server, Lync 2013, System Center, Azure, Cloud, SQL Server and more. The books are in various formats: mostly PDFs but there are EPUB, MOBI and DOCX as well.
Fair warning: these downloads seem pretty popular. You may encounter a slow server.