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Tearjerker: Before I Fall
Sam dying after she saves Juliet. To call it a Bittersweet Ending is putting it mildly. You realize everything Sam loses: her family, her friends, her future, marriage, college, kids, and everything she had to do or say with her life are gone. After spending 400 some odd pages with her. To say nothing of what her Love Interest, family, and best friends are implied to deal with grief wise.