"Zen and the Art of Motorcycle Maintenance," by Robert Pirsig.
I read it when it first came out in 1974. I was a 22-year old paratrooper with less than a year to go before getting out of the Army. I knew I was going to go to college, but I really didn't have any idea of what I wanted to study. Having gone to Woodstock and been highly influenced by the counterculture zeitgeist, I suppose I would have been an English major or something along those lines.
But ZAMM was a complete head slap. It opened my eyes to some stuff I had never thought about, and I ended up majoring in mathematics and philosophy when I finally got to college.
Although a degree in math and philosophy did not have any impact whatsoever in my future work or career paths or any other matter of consequence, it gave me a way of thinking and a foundation for exploring a variety of other intellectual paths in later years.
Many books influenced my life, but all things considered, ZAMM had the biggest influence.