Has a writer ever been more productive in death than Kurt Vonnegut? It’s a mini industry, from posthumous collections of his unpublished short fiction (“Look at the Birdie,” “While Mortals Sleep”) to the hefty Library of America volumes of his life’s work, the most recent of which, “Novels & Stories, 1950-1962,” came out in April. In October, Delacorte will release a book of his letters and Vanguard will publish “We Are What We Pretend to Be: First and Last Works.”