Tossing another penny down the well, I'd go with the Internals books. I'd especially focus on T-SQL Querying book. It has a couple of chapters on how the optimizer works before it gets into all the other stuff of writing queries.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning