Delighted to see this here. As the editor mentions, there was also a separate effort to collect opinions from American authors, including Sherwood Anderson, Pearl Buck, Theodore Dreiser, William Faulkner, Dashiell Hammett, Ernest Hemingway, Langston Hughes, and many others. The text for that is online too--"Writers take sides: letters about the war in Spain from 418 American authors": https://stars.library.ucf.edu/prism/648/