Kindred by Octavia Butler (1979)

Written in the mid-1970s and set in contemporary times, Kindred is about Dana, a black woman in her 20s who is mysteriously pulled back in time to the antebellum South to keep saving the life of a man she eventually finds out is one of her ancestors.  Dana keeps getting pulled back and forth in time--sometimes alone, sometimes with her white husband--and her life keeps intertwining with that of her ancestor and the slaves he keeps on his plantation. 

Provides a fascinating look at the lives of slaves, as well as life for any black person in the antebellum South.  In addition, it's a very interesting look at the relationship between Dana and her husband and their life in the 1970s.  Although it sounds a bit grueling, it's also compulsively readable and amazingly well told.

In the edition I read, there was a fascinating critical essay by Robert Crossley that provided more context.  Despite being so much of its time period, the novel itself does not seem dated at all.  It's still a fascinating novel with a lot to say.