Kindred, by Octavia Butler

Goodreads description:

>The first science fiction written by a black woman, Kindred has become a cornerstone of black American literature. This combination of slave memoir, fantasy, and historical fiction is a novel of rich literary complexity. 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...

ETA: Wow, I've read 36 of those (and given up on a few others). It's a really good list, but some are much longer/more dense than others. Funny seeing The Second Sex and The Old Man And The Sea on the same list. One you could finish on a long train journey, the other... you can't. Unless the train is taking you to Siberia.