This is probably the most depressing book I have ever read. Throughout the whole thing I kept hoping that something good would happen, that all the people in the book (and the whole world) were not doomed. But no... they were all doomed. It was terrible. I think my reaction contains two of the conclusions/lessons that Shute wants people to take away from the book.
First, he wants people to realize how dangerous the world is in the nuclear age. He wants people to think about the possibility of nuclear war and to fear it. He wants us to conclude that we have to change our attitudes towards national pride and war and realize that we have to live (somewhat) peacefully or we will all die.
Second, he wants us to realize what a huge capacity people have for hope, even in the most dire of circumstances. I hoped throughout the book for a happy ending. So did most of the characters. We, as humans, seem to be wired to hope for the best, to believe that things will get better. Shute is trying to point that out.