Definitely not the best, but the most mathematical fiction book might go to Surreal Numbers: How Two Ex-Students Turned On to Pure Mathematics and Found Total H

Definitely not the best, but the most mathematical fiction book might go to Surreal Numbers: How Two Ex-Students Turned On to Pure Mathematics and Found Total Happiness by of course no other than Donald Knuth. And you could put The Part-Time Parliament by Leslie Lamport near that if you stretch it.