Delvalle's plays explore historical and folkloric themes. He wrote a trilogy of plays dealing with events during the Paraguayan War, multiple plays exploring Paraguayan mythology and Guaraní folklore, and several librettos for musical comedies and zarzuelas. The situation of Paraguayan tenant farmers is a recurring theme in his work. His novel "Un viento negro" (A Black Wind), for which he won the 2013 Paraguayan National Prize for Literature, explores the terror of life under the repressive Stroessner dictatorship, which ruled Paraguay for thirty-five years (from 1954 to 1989).