I recently had the opportunity to contribute a couple of chapters to this new SQL Server security book from Syngress. The concept of the book is to provide a fundamental understanding for harried IT workers on how to use SQL Server 2005's security features to tighten down their SQL Servers. The book is intentionally broad but each author tried to put in best practices and Microsoft recommendations where possible.
I believe this is the first SQL Server 2005 security book on the market. There are a great deal of additions in SQL Server 2005 with respect to security which can prove to be a steep learning curve from SQL Server 2000. Hopefully this book serves to speed along the learning process.