The League of American Writers was a so-called "mass organization" initiated by the Communist Party USA (CPUSA) in 1935 and terminated in January 1943.
A small and elite organization, the League included professional novelists, playwrights, poets, journalists, and literary critics.
Despite the prominent role of the CPUSA in the establishment and control of the League, one should not make the assumption that any particular individual on this list was a "Communist," however.
The office of the League of American Writers was burglarized shortly before the National Council voted to terminate the organization in January 1943 and its membership list was taken.
Folsom worked from published League Bulletins, organizational minutes, partial membership lists obtained from the Federal Bureau of Investigation and the House Committee on Un-American Activities, as cross-checked to a similar list compiled by Tom Wolfe for his doctoral dissertation.