What's your favorite novel that defies genre classification?

Some of the greatest books ever written are the hardest to pin down to one genre. Are they science fiction? Fantasy? Mystery? Magical realism? Sometimes the best stories are the slipperiest in terms of labels. What's your favorite genre-defying novel of all time?

Please include a picture of the book cover or something else— and if the book cover doesn't include the title, please list that below. Thanks!

Top image: Book of the New Sun by Gene Wolfe.