The Selfish Gene by Richard Dawkins lit up for me an entirely different way of looking at living things and how Life works. And I have a degree in Biology!
Men Are From Mars, Women Are From Venus was a revelation to me. It explained why, as a man, I cannot ever fully understand women, and am never going to; none of us ever are, but that's okay. A real epiphany, as now I can just relax and enjoy them.
A Simple Path by His Holiness the Dalai Lama. Clear. Inspiring. With advice that is actually doable. The Dalai Lama has dozens of his works translated into English, and I haven't read one yet that I did not enjoy or find insightful.
All this having been said, I still maintain, after 59 years of life and thousands upon thousands of books read, the best book I ever read has been the Lord of the Rings trilogy. I have reread it many times, and it always delivers more. It is a deep pool of mythology and symbolism that is always a joy to swim in.