This is a tough question because there are so many books that I love. I don't think I can narrow it down to just one. One of my absolute favorite books of all time is Watership Down by Richard Adams. It is the story of a group of rabbits that head out on an adventure across the English countryside to find a new home after their previous home is destroyed.
Even though the story line sounds like a children's book, it is a beautifully written and moving book that is suitable for people of all ages. If you haven't read it before, I can't recommend it enough.
If you want a book that will expand your mind and are into science, I would definitely recommend The Holographic Universe by Michael Talbot. It attempts to explain the nature of reality through the use of quantum physics and neuroscience. Although it sounds like it would be a bit boring, it is anything but.
Talbot breaks down extremely complex ideas from quantum physics into easy-to-understand concepts, and then uses them to explain consciousness and the world around us. He also delves into how a holographic model for the universe could explain countless paranormal phenomenon or strange experiences that have been well documented, but can't currently be explained by science such as near death experiences, ESP, telekinesis, lucid dreaming and the effects of LSD, just to name a few. While I remain skeptical about some of the stories presented in the second half of the book, overall this book completely changed the way that I view reality, which is saying a lot.
I'm anxious to hear what other suggestions you get since I'm always on the lookout for new books to read as well!