It has an interesting premise but ignores the best parts of it. It's also far too slow going in the start, although it redeems itself in the end. Not sure you want to endure getting there though.
North Korea through human error thinks nuclear warheads are being launched at them. In response, they initiate a nuclear holocaust that devastates the earth. Far above us is the science team of a high altitude balloon. They witness the holocaust, and their supplies of oxygen are running out. Soon they'll have to land. What will they do then?
There's a reason why writers are told to start as close to the action as possible. The book begins with the North Koreans detecting what appears to be a nuclear launch. Then, thirty percent into the book, they launch their own missiles back. Thirty percent of the beginning of the book is devoted to characters and balloon jargon, and then, when the book really starts, the balloon is secondary to the action. The rest of the book isn't bad, but it's just a typical after the end story. The intriguing idea of a balloon hovering over a nuke-devastated world seems almost an afterthought.
The writing seems jerky, switching between flashback and present day in a confusing manner. A lot of it simply isn't needed. There isn't much need to spend all that time developing character or the balloon project when the ending does fine for showing people's character, and the balloon is so peripheral to the action. It really isn't till fifty percent in the book becomes interesting, but it becomes interesting in a way which makes the previous fifty percent even harder to justify. So unfortunately I have to give this book one star.