This Immortal — Roger Zelazny

Conrad Nomikos has a long, rich personal history that he'd rather not talk about. He has also been given a job that he'd rather not do. Escorting an alien grandee on a guided tour around the shattered remains of Earth after a catastrophic nuclear war is not something he relishes. Especially when it it rapidly becomes clear that he is at the centre of high-level intrigue that could determine the future of Earth itself. But Conrad Nomikos is a very special guy and he's been around - for a very long time.