Inside SQL Server 2005: The storage engine goes into a fair bit of detail on the storage engine.
The Guru's guide to SQl Architecture and internals has a lot of info on memory, storage, scheduling and other. It's for 2000, but is still fairly relevant in most places.
What specifically are you looking for?
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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