Earth Abides is a book from 1949 that has recently been republished in the SF Masterworks
The premise of the book is very simple - a plague has wiped out the vast majority of the human race. Written by George R. Stewart, a University of California English professor, the book follows Ish Williams, a geographer who returns from a field trip to discover humanity has virtually disappeared. Eventually, in San Francisco he encounters a female survivor who becomes his wife, and a small community forms around them - but rebuilding a civilization is beyond their means, and gradually they return to a simpler way of life.
A book like this one, with a plot so simple, rather plainly exposes the skill of the author - and though it goes through some rocky patches, I think ultimately Stewart pulls it off rather well. Like only the very best science fiction (and I'm using this term in its loosest sense - the book lies much closer to Brave New World or 1984 than the more usual SF fare) the book is curiously timeless, its subject matter something so universal about humanity that it has hardly aged at all. I was honestly quite taken aback to discover afterward that its more than 50 years old.
Be warned however that this is not your average page turning romp. In fact I think it's probably the most slowly paced book I've ever read. During the vast bulk of the book very little by way of events actually happens.
There is a long opening section, detached and almost poetic in feel, where the narrator essentially observes the new state of the world with humanity removed.
The middle section that follows is what stops me recommending the book unreservedly. The story has a drifting - sometimes plodding - feel to it. Many of the characters serve more as window dressing than anything else, the narrator develops some rather irritating fixations, and a few of the plot points feel a little contrived.
However the final section of the book for me redeemed all previous misadventures. It is a profound and deeply moving reflection on our societies, our civilizations, and what remains of us when we are cast back into the wild. It is also a rather beautiful portrait of old age, and the impotence of the individual in a world beyond his control.
Not many books can move me to tears any more, but this one did.
"Men go and come, but earth abides"