What was the only bestselling novel by the author who gave Hemingway his letter of introduction to Gertrude Stein?

What was the only bestselling novel by the author who gave Hemingway his letter of introduction to Gertrude Stein?

Nobel Prize winner Ernest Hemingway is seen as one of the great American 20th century novelists, and is known for works like A Farewell to Arms and The Old Man and the Sea.
Synopsis
Born on July 21, 1899, in Cicero (now in Oak Park), Illinois, Ernest Hemingway served in World War I and worked in journalism before publishing his story collection In Our Time. He was renowned for novels like The Sun Also Rises, A Farewell to Arms, For Whom the Bell Tolls, and The Old Man and the Sea, which won the 1953 Pulitzer. In 1954, Hemingway won the Nobel Prize. He committed suicide on July 2, 1961, in Ketchum, Idaho.

Gertrude Stein (February 3, 1874 – July 27, 1946) was an American novelist, poet, playwright, and art collector. Born in the Allegheny West neighborhood of Pittsburgh, Pennsylvania, and raised in Oakland, California, Stein moved to Paris in 1903, and made France her home for the remainder of her life. She hosted a Paris salon, where the leading figures of modernism in literature and art, such as Pablo Picasso, Ernest Hemingway, F. Scott Fitzgerald, Sinclair Lewis, Ezra Pound, Sherwood Anderson and Henri Matisse, would meet.[1]