Which 1929 novel introduced the fictional detective Ellery Queen?

The 1929 novel that introduced the fictional detective Ellery Queen is The Roman Hat Mystery.

The book was the first mystery novel written by the American writer-producers Frederic Dannay and Manfred Bennington Lee, who published together under the pseudonym Ellery Queen. It begins a long-running series featuring Ellery Queen, an amateur detective who assists his father, Inspector Richard Queen, of the New York Police Department.

The plot centers on a murder in New York City's Roman Theatre. The victim is found dead in a locked box during a performance, while his top hat has disappeared. Ellery applies close observation and logical deduction to the unusual crime, establishing the puzzle-centered style associated with the series.

The title is sometimes confused with later books in the same sequence, including The French Powder Mystery and The Egyptian Cross Mystery. The Roman Hat Mystery came first and launched one of America's best-known detective-fiction partnerships.