Math! War. Rarefied intellectuals, often gay men, confronting the first world-war. And trying to solve one of the long-standing mysteries of mathematics: the Riemann hypothesis. I enjoyed the varied characters, many unfamiliar, and the alien feel of the setting, even though it's not that far away or long ago. It did take me a long time to finish, just being interrupted by other things, and shorter books. I'm glad I read it.