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: