Kindred by Octavia E. Butler
Having just celebrated her 26th birthday in 1976 California, Dana,
an African-American woman, is suddenly and inexplicably wrenched through
time into antebellum Maryland. After saving a drowning white boy
there, she finds herself staring into the barrel of a shotgun and is
transported back to the present just in time to save her life.
During numerous such time-defying episodes with the same young man,
she realizes the challenge she’s been given: to protect this young
slaveholder until he can father her own great-grandmother.