Two instances come immediately to mind:
I couldn't stomach finishing
Jack London's A Daughter of the Snows.
Wikipedia notes: "Modern commentators have criticized the novel for its approval of the main character's view that Anglo-Saxons are racially superior." - source: Cassuto, Leonard; Reesman, Jeanne Campbell (1998).
Rereading Jack London. Stanford University Press. p. 161. ISBN 9780804735162.
I did finish
John Grisham's novel, A Time To Kill (1988), although I was shocked by the words and images he used to describe the black defendant, Carl Lee, padding toward him, barefoot, black, leathery soles slapping against the floor (or some words to that effect, which evoke images of an animal, not a man). Although I read the book years ago, when it was first released, I distinctly remember rereading that passage in an effort to give Grisham the benefit of the doubt - i.e., to discern whether Grisham was describing the prisoner through the lens of the culture depicted in the book, or through his own. Unfortunately, I concluded (reluctantly): the context in which Grisham cast that black character as subhuman reflects more on the author (who hails from Mississippi) and the way he was conditioning the reader to view the character, not on the redneck racists in the fictional Mississippi community where the book's set.
[EDITED to italicize
A Daughter of the Snows, which I initially punctuated as if it were a short story.]