Antifragile by Nassim Nicholas Taleb. Man, reading this book really put a fire under my ass. I realized how much more I could be getting out of life by pursuing optionality and using the barbell strategy.
A quote that stuck with me the most is: "If you have more than one reason to do something, don't do it." That means that you are trying to convince yourself to do it. Obvious decisions require no more than one good reason.