Optimization/Performance blogs

SQLskills Blogs: http://www.sqlskills.com/blogs.asp
Kimberly Tripp
Paul Randal
Bob Beauchemin
Simon Sabin
Elizabeth Vitt
Stacia Misner

Also: Whitepapers: http://www.sqlskills.com/whitepapers.asp

Craig Freedman (query plans)
http://blogs.msdn.com/craigfr

Conor Cunningham
http://www.sqlskills.com/blogs/conor

SQL Server Engine Tips
 http://blogs.msdn.com/sqltips 
SQL Programmability and API Development Team Blog
 http://blogs.msdn.com/sqlprogrammability
The SQL Server Relational Engine Manageability Blog
 http://blogs.msdn.com/sqlrem
Tips, Tricks and Advice from the SQL Server Query Optimization Team
 http://blogs.msdn.com/queryoptteam/archive/2006/04/12/575469.aspx 
Engine Watch
  http://blogs.msdn.com/weix
Ian Jose's WebLog
 http://blogs.msdn.com/ianjo
Slava Oks's WebLog
 http://blogs.msdn.com/slavao
Have Data Will Travel
 http://blogs.msdn.com/rogerwolterblog
Linchi Shea in RSS
 http://sqlblog.com/blogs/linchi_shea
Grumpy Old DBA
 http://sqlblogcasts.com/blogs/grumpyolddba (esp. Analyzing Indexes)

Books:

SQL Server 2005 Practical Troubleshooting: The Database Engine
 Editor: Ken Henderson

Inside Microsoft® SQL Server(TM) 2005: Query Tuning and Optimization
 Delaney et al, includes Craig Freedman on Query Plans, Lubor Kolar 
 

