I will be attending an advanced SQL Server training in a couple of weeks and the instructors recommended some pre-reading.
I'm sharing here a subset of those recommendations that point to articles and white papers published by Microsoft, grouped by category:

Database Storage

Tempdb

Indexes and Query Optimization

Profiling

Backup and Reliability

Security

High Availability

Replication

Programmability

Service Broker

Integration Services

New in SQL Server 2008