One of the all-time great post-apocalyptic novels. The other one is Stephen King's THE STAND, and both come to largely the same moral conclusion, though by following completely different roads.
In Swan Song, the world has been obliterated by nuclear war. It is never clear whose "fault" it all was, nor is that really of import. What matters is what happens to the survivors, small groups of people who gradually draw into two types. And unlike most post-apocalyptic novels, it isn't so much the "good" versus the "evil" as it is the "barely surviving" versus the "evil." Evil here is in the form of armies that march across the land, raping and pillaging in order to maintain their own forces. One of them in particular, the Army of Excellence, is at the core of the story. Run by a has-been Air Force colonel who found a second chance to matter when the hail of fire fell from the sky, and a young man with a warped sense of reality (he basically believes he is living his own version of a Dungeons and Dragons game), it epitomizes everything wrong with the world, not just now, but all the bad choices that led to the nukes in the first place.
Then there are the others, those who go from place to place, hoping desperately to find somewhere untouched by the nuclear winter, a place with good water and food to eat. And all in vain.
Or so it seems. Because one of the itinerant scavengers is a girl named Swan. And it seems she not only may have the power to save herself, but to bring the world to rebirth.
Of course, because Swan is essentially a Christic figure with no observable character flaws, so author McCammon heightens the stakes by giving her a Devil to contend with: a thing that looks like a man, but can change his shape at will, has super-strength, and a host of other powers that enable him to walk the world, spreading hopelessness and crushing the will to live wherever he finds it.
Swan Song, without becoming preachy, becomes a treatise on the forces of good versus evil, on the powers of light and darkness. There are a few problems with the technical side of the writing - not least of which that McCammon switches points of view often and seemingly without reason or rhyme, leading to some confusion to readers - but they are easily forgiven in light of the book's overall value as a story... and more than that, as a Tale. Adult content, so be warned, but HIGHLY recommended.