Rich Dad, Poor Dad
As a person that grew up in a lower middle-class family, this is one of the most seminal books in my personal development. If you take one thing from this book, it's Robert Kiyosaki's "Rich Dad's" advice, "everything has a price, and that price is almost never paid for in money." This lead me to see the entire world as a game of cause and effect, a game of balance, where I needed to know the price of things and understand if I'm willing to pay it. Many people never succeed because they chase things they're unwilling to pay the price for, and live unhappy with the result.
Robert T. Kiyosaki, Sharon Lechter