Free book available.

Execution plans tell you everything you need to know about what data is processed. By knowing what you should look for, you can improve your query plan and therefore improve overall performance.

http://www.red-gate.com/library/sql-server-execution-plans-2nd-edition?utm_source=ssc&utm_medium=publink&utm_campaign=books&utm_content=executionplans_ebook&utm_term=executionplans