Some book suggestions for performance tuning
I like Kalen Delaney's internals book:


Microsoft SQL Server 2008 Internals


I also like the Brent Ozar / Christian Bolton book:


Professional SQL Server 2008 Internals and Troubleshooting


For T-SQL tuning, you can't get much a better T-SQL wizard than Itzik Ben-Gan:


Inside Microsoft SQL Server 2008: T-SQL Programming
Inside Microsoft SQL Server 2008: T-SQL Querying


And query tuning overall, Grant Fritchey and Sajal Dam:


SQL Server 2008 Query Performance Tuning Distilled


There are also some interesting solutions and opinions presented in various chapters in the two MVP Deep Dives titles (from which all proceeds go to two charities, War Child and Operation Smile):


SQL Server MVP Deep Dives Volume 1 (War Child)
SQL Server MVP Deep Dives Volume 2 (Operation Smile)


And a couple of the Red Gate titles @Kev left out, but which are both excellent:


Inside the SQL Server Query Optimizer (Ben Nevarez)
Troubleshooting SQL Server: A Guide for the Accidental DBA (Jonathan Kehayias / Ted Kreuger)