How many books of poetry did the poet, who was published by Sheldon Dick, write?
Sheldon Dick: Sheldon Dick (1906–1950) was an American publisher, literary agent, photographer, and filmmaker. Sheldon Dick:  He published a book by poet Edgar Lee Masters, and made a documentary about mining that has been of interest to scholars. Edgar Lee Masters:  In all, Masters published twelve plays, twenty-one books of poetry, six novels and six biographies, including those of Abraham Lincoln, Mark Twain, Vachel Lindsay, and Walt Whitman.
twenty-one