Understanding the psychology and science behind marketing and merchandising can make it easier to resist impulsive and unnecessary purchases. These books are an excellent place to start.
Why We Buy: the Science of Shopping by Paco Underhill
Buyology: Truth and Lies About Why We Buy by Martin Linstrom
Buying In: The Secret Dialogue Between What We Buy and Who We Are by Rob Walker
Predictably Irrational: The Hidden Forces That Shape Our Decisions by Dan Ariely
The Complete Tightwad Gazette by Amy Dacyzyn
Your Money Or Your Life by Vicki Robin, Joe Dominguez and Monique Tilford
Mindless Eating: Why We Eat More Than We Think by Brian Wansink