Microsoft offers a new free eBook for download.
You can download the eBook “Introducing Windows Server 2012 R2 Preview Release”.
You can download it here.
With day 1 and right to the keynote from Ben Armstrong, the preview of Windows Server 2012 R2 and System Center 2012 R2 is available.
You can find them in the TechNet Evaluation Center:
Download Windows Server 2012 R2 preview
Download System Center 2012 R2 preview
Download System Center 2012 Configuration Manager and Endpoint Protection Preview