Poul Anderson 
Byworlder 

Early in the 21st century the world is enjoying an uneasy peace, with a distinct division between the "straight" society and the various fringe groups that go to form the Byworld.Tension grows, however, over the presence of an alien spaceship that is orbiting the world, bearing a single occupant – the Sigman. It appears that no-one knows how to communicate with the Sigman; no-one knows the purpose of his visit. Until two people – one "straight" and the other a Byworlder – solve the problems involved; and in so doing trigger off a series of violent plots and counterplots that mount to a frenetic climax.