Annie Jacobsen's _Nuclear War: A Scenario_
A second by, at times, millisecond, tale of what could and would happen after a nuclear attack on the US is detected. Been a long time since I read a book in just over two days.
If you haven't seen the tv show Madam Secretary (minor spoiler warning), there's an episode that more or less films this scenario playing out. You can find clips on YouTube of just the "nuclear war is about to happen" part of the episode, and I highly recommend it as a dramatization of this kind of thing. (It's a great show in general, too.)