One of the Hugo nominees I had not read.
The fascinating story of a communicable disease, sort of. People are afflicted (?) with a map of a fragment of a city (known as Palimpsest)in a parallel dimension (or in another world, or something) as a mark on their skin. The map is a sexually-transmitted disease. But everyone has a different fragment of the city. When you fall asleep after having sex with someone with a map, you enter the city in your dreams. But the things that happen to you there (like losing fingers) have an effect in the "real" world.
When you first enter the city, you're grouped with 3 other people. You can only enter the city permanently by finding the other three in the real world.
The way people deal in the everyday world with the effects of the disease is quite interesting. Palimpsest is drawn mostly in fragments (perhaps echoing the marks on the characters' skin).
One rare element is man-on-man sex. Tastefully drawn, to be sure, but much more rare, in general, than its lesbian counterpart (which also appears).