SIlvano Coriani has done a great job compiling the most relevant SQL Server whitepapers available on SQL CAT and Technet sites. These papers are categorized into the major categories of Performance and Tuning, High Availability and DR, Security, Manageability, Migrations and Upgrade and Business Intelligence. The papers are a great resource for customers who want to improve their knowledge of specific and targeted areas in SQL and Windows Server quickly.
Performance and tuning
High Availability and Disaster Recovery
Migrations and Upgrade