I've got a tie between two books:

1) Design of Everyday Things by Dan Norman

This book ruined my life. I highly recommend it. Every engineer, manager and designer should read this. Maybe every human. I think of this book every time I try to pull a push door, every time I reach the bottom floor of a stairwell and notice the design that might save my life one day, and every time I try to struggle to operate a television or a microwave.

2) Influence: The Psychology of Persuasion by Robert Cialdini

This book helped me understand myself and everyone else. For example, I now understand why I double down on dumb ideas. I also catch a lot more marketing and sales tricks.

Edit: Sorry, just now realized that I broke the 1-book rule, but it's probably too late to correct this and it's really hard to choose between these two anyway.