1. Thinking Fast and Slow - Daniel Kahneman. If you're going to read one, read this. A lot of theory undergirding how people think. Decision making by people will make a lot more sense after this.
2. Influence - Robert Cialdani. Less theory and more pragmatic advice on how to influence people.
3. Drive - Daniel Pink, Switch - Chip Heath, Made to Stick - Chip Heath, Predictably Irrational - Dan Ariely. Books on specific sub-categories. More pop-psych. Information density is less, but easier to read.
4. Poor Charlie's Almanac - Charlie Munger. Best known as Warren Buffett's partner, this book is a collection of his speeches, letters, etc... You get an idea of how he thinks but, but you have to dig through the repetitive ramble to get it. Think of it as Charlie observing a lot of the prior principles but putting it into a real life/business context.