If you want to learn things about SQL Server that you'll never find anywhere else, you must read Ken Henderson's Guru's Guides to SQL Server. I just got his most recent book, The Guru's Guide to SQL Server Architecture and Internals
, and like his previous books it is amazing. This one is totally packed with everything that wasn't included in the earlier volumes: how SQL Server uses WinAPI calls, thread scheduling, virtual memory management, how the different network libraries operate, how to “debug” the internal processes, how to discover undocumented calls, more XML not previously covered, Notification Services, replication, and things you'd absolutely NEVER think DTS could do, but can. Tons of cool stuff you can do with COM and ODS, and an EXTREMELY DETAILED look at how the SQL Server query processor functions. Probably his best book so far, which is saying a great deal. Highly recommended.