This is one of those books that is hard for me to classify as either sci-fi or fantasy.
It's deliciously subtle and simple and complex, all at the same time. Kit is a bridge builder who has come to build a bridge across the mist. This mist "river" slices the empire nearly in half. The only way to cross the mist is by ferry, which is more than a little treacherous. The mist is dangerous and mysterious. Its properties are neither thoroughly explained, nor really Known. There are dangerous creatures that live in the mist and it seems to be largely unpredictable.
This story isn't about the mist, though, although it is almost its own character. The story is about Kit and not just his building of the bridge, but his life, his character and his relationship with the townsfolk and other workers, especially the ferry-woman Rasali, whose livelihood (and love) his bridge will destroy. I love the townsfolk, the characters, their feelings and acceptance of life. I love the environment and the weirdness and zen of it all.
This work is complicated and deep, gentle and sophisticated. I've only read a couple of Johnson's other works and she's just a consistently phenomenal writer. This doesn't have the emotional sucker punch of last year's Ponies, but it's just as strong and beautifully done.