best non fiction book on any subject?

I think I've reached the point in life where I'll happily read a brilliant book about almost anything.

Gardening? Great. Car repair? Sure. The history of agriculture? Absolutely. If someone tells me there's a 500-page masterpiece on concrete, medieval tax collection, or the evolution of staplers, I'm in.

So what's the best non-fiction book you've ever read? I don't care about the subject, I just want books that are so well-written, that they make you obsessed with a topic you never knew you cared about.

Hit me with your favourites.