RealTimePublishers has a whole bunch of free ebooks:
Database
The Definitive Guide to Scaling Out SQL Server 2005
The Definitive Guide to Scaling Out SQL Server 2000
The Essentials Series: Optimizing Database Connection Performance
The Shortcut Guide to SQL Server Infrastructure Optimization
Few other ones that might be of interest:
The Administrator Shortcut Guide to VB Scripting for Windows
The Definitive Guide to Building a Windows Server 2008 Infrastructure
The Definitive Guide to Windows Server 2003 Terminal Services Updated Edition
The Shortcut Guide to Selecting the Right Virtualization Solution
The Shortcut Guide to Virtualization and Service Automation
Enjoy!