With the recent release of Windows Server 2008 R2, Microsoft has released an eBook for download on the Windows Server website.  Although not a huge tome, it has some good content, including:

• What's new in Windows Server 2008 R2
• Hyper-V R2
• Remote Desktop Services (RDS) and VDI
  
• Active Directory: Improving and Automating Identity and Access
• IIS 7.5
• DirectAccess

And the best part - it's free.  Get it here from Microsoft, or below:

Download: Introducing Windows Server 2008 R2 (11.1 MB)