Please recommend #books about #maths that *aren't* about teaching it.

I'm looking for books that someone might sit and read for pleasure rather than for academic reasons (so not really textbooks, but I guess there might be some really compelling textbooks out there). Fact or fiction. Feel free to define "about maths" however you like (there may be points for really inventive interpretations). They might only be loosely about maths, but I'll set the criterion that someone who thinks they're not into maths would recognise it as maths.

Go!