What literary work do scholars usually say signals the beginning in harlem renaissance?

The Souls of Black Folk by W.E.B. DuBois

That work by DuBois was published in 1903, and set the tone for what was to follow.  Other key works later were The Negro's Greatest Enemy by Marcus Garvey (1923), The New Negro: Voices of the Harlem Renaissance, edited by Alain Locke (1925) -- which featured his article, "Enter the New Negro," as well as selections by other writers in the movement -- and Their Eyes Were Watching God by Zora Neale Hurston.  And those are just a few samples among many great works in the Harlem Renaissance movement.