Chaos by James Gleick changed how I look at the world; stock market, weather, everything.
Happiness is a Serious Problem by Prager--short read, fascinating ideas
Forgive to Live by Tibbets
Essentialism by Greg McKeown, pretty mustachian, has not changed me as much as I'd like YET but it's nibbling away at me
The Screwtape Letters by CS Lewis; grown up spirituality in a deceptively simple short book
Guns Germs Steel, gives an overarching theory of human evolution that is thought provoking and has good explanatory power
Good Calories, Bad Calories by Gary Taubes, reversed decades of medical propaganda (low fat, grain, vegeterian, etc) in my medically trained mind in a few days. Long read; "Why We Get Fat" is an easier read for the less medically inclined.)
The New Atkins for a New You, started me on Low Carb (I am a physician and I now HIGHLY recommend low carb high fat eating)--an excellent how to do it book with a fair amount of theory by highly experienced nutritionists and physician/reserchers
Body By Science by Doug McGuff MD, changed my exercise forever. I now lift heavy weights to failure once each one to two weeks and I'm now, at 62 years old, doing 205 lb pulldowns. Amazing.