Predictably Irrational: The Hidden Forces That Shape Our Decisions

Ariely's contribution is the word "predictably": we are not randomly irrational but systematically so, and the patterns of irrationality are stable enough to study and to design for. The book walks through experiments on relativity in pricing, the cost of zero cost, the effect of expectations on experience, and the power of social norms versus market norms. For product direction the most useful chapters are the ones on free (why "free" changes behaviour categorically, not just by reducing price to zero) and on the gap between what people say they will do and what they actually do. Read alongside Kahneman for the cognitive architecture and Lewis's Impulse for the consumer-behaviour complement. Ariely writes accessibly; the experiments are memorable.