It’s a fantasy in an alternate world that does have some overlap with Lovecraft. But in that world Johnson weaves a story that is partly David McCullough feat of engineering story and partly a love story. Lovecraft was fond of vast onyx castles and the like, but he never dwelled on the experience of building one. It certainly doesn’t appear to have occurred to him to write a love story. These genres mix surprisingly well.
The nation in this world is divided by a mist river that corrodes most materials except for metals, certain woods and ropes made from the skins of the fish that swim in it. The mist is denser than water and caustic. Deep under the river of mist live the Big Ones, largely unseen but deadly creatures. The mist can only be crossed on light skiffs, and the passage is dangerous. An engineer is brought there to build a bridge across the river.
In excellent prose, Johnson tells a story that is wise about the ways that all technological advances shape society and its effects on individuals. What happens to the skiff pilots when the bridge is finished? Is connecting the two halves of the kingdom a good thing? It seems likely, but is it worth the cost? There is a real understanding of the satisfaction of a job well done, that is counterbalanced by a real understanding that every advance is a leaving of something else, for good and ill. There is a real understanding of economics. The love story pays off in a believable way; emotional but not naive about the future. It evokes that sense of divine discontent that Kenneth Graham talked about in The Wind and the Willows*; a childlike awe without being childish. It is a perfect story all round.
*In addition to this and the River of Bees collection, if you grew up on Wind in the Willows, Johnson's sequel to that The River Bank is also excellent!