Final answer:
Washington Irving's first novel was 'The Legend of Sleepy Hollow', published in 1820.
Step-by-step explanation:
Washington Irving's first novel was The Legend of Sleepy Hollow, which was published in 1820. This novel is a classic piece of American literature and is known for its portrayal of the supernatural and the iconic headless horseman. It tells the story of Ichabod Crane's encounter with the headless horseman in the fictional town of Sleepy Hollow.