The book I've read most often is The Little Prince, a short novel by the French writer Antoine de Saint-Exupéry. I'm a pharmacist in Guangzhou, and I think I've read it at least five times, in Chinese and once in English.
I first read it when I was ten, because my aunt, who was a primary school teacher, gave me a copy with the original watercolour illustrations. The story is narrated by a pilot whose plane crashes in the Sahara Desert. There he meets a strange little boy who says he comes from a tiny planet, where he looked after a single rose. The boy describes visiting other small planets, each with one odd adult living alone, like a king with no subjects and a businessman who counts stars. Eventually he meets a fox on Earth who teaches him about friendship.
I keep going back to it because it seems to change as I get older. At ten, I read it as a fairy tale about a funny boy and talking animals. When I reread it at university, I found the adults on the planets hilarious, because they reminded me of my professors. And when I read it again last year, after my first child was born, I found it quite sad, because it's really about how adults forget what matters. The fox says something about how you become responsible for what you love, and that struck me differently as a new father. It's also short, so I can read it in one evening. I suspect I'll read it again when my daughter's old enough, and I'm curious how it'll feel then.