In New York Fles first worked as a violinist, painting apartments, selling vacuum cleaners and for publishers. In 1933 he started his own business, a "literary agency", in Manhattan, New York. As a "literary agent" he mostly helped writers find publishers. He also sold stories for films or magazines and would find a writer to make a book that a publisher wanted. When Barthold started working as a literary agent, many of his clients were people who left Germany for the United States because of the Nazism in Germany. He held meetings for these writers in New York so that they could learn about publishing books and stories in the United States. Later, however, most of the writers he worked with were born in the United States.