I've been working with SQL Server for 11 years and run SQL Server Central.
And I learned a bunch from this book. This is one of the best references on SQL Server Security that I have seen written and I recommend it highly to every SQL Server DBA.
The book is written to cover versions 6.5, 7.0, and 2000. And it does a great job with each. It starts by looking at the way that logins are authenticated by the server. Great detail is given, even to the point of examining network sniffer traces to show how the communication occurs between the client and server.
From there, the database security is examined with separate chapters for v6.5 and 7/2000 since they work differently. Not only is the process explained, but the author notes where there are bugs and unforseen consequences of assigning security in certain ways.
The early chapters provide insight into how security works in SQL Server. The later chapters build on this to give hints and suggestions for implementing security in your applications, DTS, replication, and even SQL Server CE.
Overall, this is a must read for SQL Server DBAs. Developers will benefit as well since a thorough understanding can solve a great many problems and prevent even more.