This fascinating short novel about the Watchers, a group that sees to the welfare of many alternate versions of Earth, sends two feature story journalists... poetess Elspeth, photographer Mack... as apprentice agents to two versions of North America. The first features a North America remade during the turbulence of the Napoleonic Wars... a conservative, Southern-dominated country (Columbia) set up by Aaron Burr and his allies after they overthrew the Madison Administration and a Mexican empire established by Napoleon himself after a Columbian frigate sprang him from exile in Saint Helena. The second is a somewhat fictional version of our own world, set slightly ahead of the novel's publication (with a third Roosevelt in the White House who splits his time between Washington and the emerging world capital of San Francisco, which retained the United Nations).
Though the novel spends too much time on Elspeth's and Mack's flings and their own extended flirtation, it also has many strengths. Merwin presents a series of vivid characters from Columbia (e.g., an Afro-Columbian general with a suspicious resemblance to Othello who is Elspeth's love interest, an inventor haunted by his family's techno-carpetbagging in a post-alternate Civil War New York City, a vicious agent of a foreign power) and "our" world (the team's supervising Watcher agent Juana who is Mack's love interest, President Roosevelt and his daughter Christine). I think the novel's greatest strength is how Merwin manages to sell each of the North Americas... including the lightly-sketched Brittanic one from which Elspeth and Mack originate... as full equals, each with its quirks, strengths and weaknesses.
At a panel discussion I attended years ago, Harry Turtledove and the other SF authors on the panel gave high praise to "House of Many Worlds" as a classic of alternate worlds SF.