Rhodes' earlier "The Making of the Atomic Bomb" won a Pulitzer prize; I thought this was even better. The first part is an account of Soviet espionage into the Manhattan project; Rhodes lets us in on all the mundane details while allowing the inherent drama to come through in full force.
The second part was even more of a revelation: I never thought the nature of the "technically sweet" innovation that saved the H-bomb project would be revealed to the public during my lifetime, but it's spelled out here. I also never thought I'd understand in detail how an H-bomb works, but Rhodes makes it both comprehensible and fascinating.
Chapter 24 is the heart of the book--a description of the Mike shot, the world's first thermonuclear explosion. Don't start reading it if you have to go somewhere soon. A classic case of "I couldn't put it down".