Gaudy Night — Dorothy L. Sayers

When Harriet Vane attends her Oxford reunion, known as the "Gaudy," the prim academic setting is haunted by a rash of bizarre pranks: scrawled obscenities, burnt effigies, and poison-pen letters -- including one that says, "Ask your boyfriend with the title if he likes arsenic in his soup." Some of the notes threaten murder; all are perfectly ghastly; yet in spite of their scurrilous nature, all are perfectly worded. And Harriet finds herself ensnared in a nightmare of romance and terror, with only the tiniest shreds of clues to challenge her powers of detection, and those of her suitor, Lord Peter Wimsey.