Influence
once you've read this, you won't believe how often you've been persuaded by these 'tricks'.
The full title is Influence, The Psychology of Persuasion and it's one of the trio of classic strategy books. Like the other two, the basis for this book is evidence from experiment. This book in particular reveals the six tactics commonly used to influence people and explains how they work.
Cialdini is a charming and interesting writer, so this book is a much more enjoyable read than your average strategy or business improvement book.
Like the other books in the ‘trio’ (alongside Good to Great and Tipping Point) the advice in this book (although updated) really does stand the test of time. The principles such as likability, social proof, consistency and authority are still fundamental considerations if you want to persuade people.