Belle's sql musings blog has a list of 7 free SQL Server books.

The books are the following ones

SQL Server 2008 MS Press Free Ebook
How to Become an Exceptional DBA
Best of SQL Server Central
Brad’s Sure Guide to SQL Server 2008
Brad’s Sure DBA Checklist
RedGate’s SQL Server DBA Best Practices
Dissecting SQL Server Execution Plans

These books are offered by Microsoft Press, RedGate and SimpleTalk

Get the links to all these books here: