This is, I understand, Mr. Conroy's last novel. It is not one of his alternate history novels but a disaster novel. It's OK. It didn't grab as did his other novels. It follows his usual template. Boy meets girl and falls in love amid turmoil of history/disaster. As I said I liked it. If I had to give it a number between one and five, it would be a three and a half.
UPDATE: NOT his last. At least one more to come in June from Baen. It's co-authored by J. R. Dunn and titled The Day After Gettysburg.