Eric Ligman from Microsoft has posted a collection of 100+ free eBooks for download.
Topics include:
Windows 8.1
Windows 8
Windows 7
Office 2013
Office 2010
Office 365
SharePoint 2013
Dynamics
PowerShell
Lync
Exchange
Azure
SQL
…And More
Grab them from the MSDN blog post: Largest Collection of FREE Microsoft eBooks Ever