Kindred - Octavia E Butler

Summary:

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.

Summary from goodreads.com