Farnham's Freehold by Robert A. Heinlein ISBN 0-671-72206-9
This old novel (copyrighted 1964) is an unusual sort of sci-fi. It not only deals with time travel on earth but also deals a blow to the issue of racism, where one race puts itself above all others.
It starts with 20th century USA, in the home of Hugh Farnham. There are six adults, five whites (Hugh, his wife, son, daughter and her female friend) and one black servant Joe, a college student. Just as world War III takes off, they move into Hugh's bomb shelter underground. One final atomic bomb hits the vicinity of the home, displacing their bunker in time by about two thousand years.
They crawl out of the shelter not to meet destruction but what looks like virgin semi-tropical land. For months they survive on the land, gardening and hunting and fishing. When the two young ladies announce they are pregnant (with conception presumably before the bomb attack), it is mostly received as good news, as it would "broaden" the gene pool of the human race. Or so they thought. They were assuming they were the only humans alive.
Shortly after the tragic loss of one of the ladies (Hugh's daughter) during prolonged labor, followed by the unhealthy baby the next day, the story takes a sharp new turn. Our survivors are visited by other people arriving in what looks like a spaceship. It turns out the Farnham clan had been trespassing on these people's land. The Farnhams are taken prisoners without a fight, as these new people have superior technology that would have made any effort to fight back tantamount to suicide.
In this new world, black people reign supreme and pure whites are genetically-controlled slaves. Blacks are the "Chosen" and so formerly-lowly Joe is automatically given preferential treatment and accepted as one of them, while Hugh and the rest of his family are enslaved. Hugh is made a special servant, to translate the books he'd stocked in the bomb shelter into the new language of the people, a language that seemed to him to have roots in African and Asian languages of his time. He tries in the process to piece together what had happened to the world he used to know.
And just as black readers (like me) would start enjoying the sudden elevation of their race, we are given yet another jolt. The rulers are cannibals! Like the early Europeans regarded the natives of the Americas and Africa as sub-human, they considered whites as low as animals. Even to the point of getting them bred and fattened for meat euphemistically called "pork," along with pigs! This discovery by Hugh leads him to plan for escape with his new family of three (his daughter's friend and their twin boys) to the mountains where the "savages" live free of slavery. (His wife and son had gone "native," being treated like pets in the master's house and would have nothing to do with him.)
Hugh's plan is short-lived; they are caught before they could even leave the premises. They are taken back to a period of captivity, at the end of which they are given some difficult options to choose from. None of the options suit them, so then rather than have them killed, the lord of the mansion sends them back to their own time, back to 20th Century USA on the night of the bombing. Or so it seemed...
In the final moments, Hugh had to contemplate the issue of racism, later with his new wife. Hugh had earlier recalled a place called Pernambuco where the rich plantation owners where French-educated blacks and the servants and field hands mostly white. And hear this:
But Hugh knew that the situation was still more confused. Many Roman citizens had been "black as the ace of spades" and many slaves of the Romans had been as blond as Hitler wanted to be -- so any "white man" of European ancestry was certain to have a dash of Negro blood. Sometimes more than a dash. That southern Senator, ... the one who had built his career on "white supremacy" ... had had many transfusions -- and his blood type was such that the chances were two hundred to one that its owner had not just a touch of the tarbrush but practically the whole tar barrel.
Their eventual conclusion: Racism and abuse of power has nothing to do with color. Anyone can do it.