Hell to Pay: Operation Downfall and the Invasion of Japan by Giangreco is a comparison of the American plans for the invasion of the Home Islands with the Japan

Hell to Pay: Operation Downfall and the Invasion of Japan by Giangreco is a comparison of the American plans for the invasion of the Home Islands with the Japanese plans for their defense. It talks through the cost in live of such an invasion. Though it doesn't explicitly speak to the value of the Manhattan project, it argues for it indirectly by explaining the horrifying brutality that would have been that invasion.