Most people will answer that question with a list. They can't choose just one and I understand why. There are a lot of great books. How could you choose just one?
That's why I feel compelled to answer. While there are many excellent books, there is one in my mind that stands above the others. It addresses the big questions, life, death and love. If I met the Dali Lama, I would say "your holiness, have you read this book?" This is the opening paragraph:
Late in the winter of my seventeenth year, my mother decided I was depressed, presumably because I rarely left the house, spent quite a lot of time in bed, read the same book over and over, ate infrequently, and devoted quite a bit of my abundant free time to thinking about death.Do you recognize it? It's The Fault In Our Stars by John Green. Five stars and 10,137 reviews. Yes, it will make you weep, but that's the beauty of this magnificent work. It will break your heart so that it's never closed again.