Dan Ariely's Predictably Irrational has probably had the most profound impact in changing the way I view how people behave the way we do. The title says it all: rather than perfectly rational creatures, in many ways we are irrational, but in predictable ways. Ariely's point is that if we understand these ways we are irrational, we can use them to overcome the limitations of that irrationality, and his book gives practical and humorous examples and suggestions.
He's a behavioural economist (yeah, I didn't know there was such a thing either) so his research focuses on consumer and business choices - from purchasing decisions, retirement planning, health care, executive bonuses - but that covers a wide area, and there's a lot to be learned about human behaviour in general.
I first read Predictably Irrational a couple of years ago and have annoyed friends since by quoting it in appropriate (to me) circumstances. I am finishing up his follow-up, The Upside of Irrationality, now. When I had the opportunity to meet one of my favourite authors, Yann Martel (Life of Pi), at the Vancouver writers festival, I babbled to him about Ariely's research as it applied to Martel's Beatrice & Virgil ... because I was a volunteer writer escort who had to wait until he was done a book signing to escort him back, so I was reading on my iPhone, and on the way back we talked about the impossibility of signing an e-book which is how I bought B&V, which made him comment on me reading a book on my iPhone, which happened to be The Upside of Irrationality and happened to be a chapter that touched on themes of Martel's talk and quoted Stalin in the same way Martel had just quoted: "The death of one man is a tragedy. The death of a million is a statistic." He at least pretended to be interested.
Anyway, below is a taste of Dan Ariely's humour and insight while giving a TED talk. You really have to read this book though.