The Hammer of God, written when Clarke was in his mid-70s, is one of Clarke's absolute best books. It tells the story of a huge planet killer of a rock on its way through space on a collision course with Earth, and the efforts made by humanity to prevent the collision (efforts that are sabotaged by religious fanatics who are convinced that the asteroid, named Kali, is quite literally the "hammer of God," sent to judge mankind for its sins). The Hammer of God has everything you could ever want in a novel: a good story, some suspense, a little action, interesting characters, wit, humour, sex, comments on religion (Clarke has even invented a new religion, called "Chrislam"), and such a wealth of ideas that it would be sufficient for a whole series of novels. It was optioned by Steven Spielberg (who later made "Deep Impact," without any mention of Hammer of God in the credits, though).
The subject matter concerns something that is highly relevant in our time and potentially more important than anything else: the danger from "planet killers," that is, comets or giant asteroids colliding with Earth. This danger has always been pretty much ignored (usually because of complete ignorance) by both common people and, which is a lot worse, the people in power, who really should know better. This book has helped raise the awareness of the danger, and, besides, it's a very enjoyable read. Very highly recommended.