Kindred

Butler gives the time travel no machine and no scientific explanation. Dana is simply summoned whenever Rufus Weylin, the son of a Maryland plantation owner, is in mortal danger, and she returns home only when her own life seems threatened. She soon realizes that Rufus is her ancestor and that her own family line depends on him surviving long enough to father a child with Alice Greenwood, a free Black girl who grows up near the plantation.

Each trip lasts longer in the past, while only minutes or hours pass at home. Dana must pretend to be enslaved, learn to read the dangers of the household and protect people she comes to care about, while trying to influence Rufus as he grows from a frightened child into a man shaped by the power the system gives him. When Kevin, who is white, is pulled along on one visit, he is stranded for years before she can bring him back. The violence of slavery is present throughout and handled seriously but without sensationalism; what lingers most is the way ordinary people were forced into impossible choices.

Intense, clear and very hard to put down. About 287 pages in the Beacon Press anniversary edition, with a reader's guide.