His most important volume of sociology, "Burghezia română" ("The Romanian Bourgeoisie"), was published in 1925. Two years later, he became a philosophy professor at the University of Iași. His last book, "Nirvana" (1928), deals with his understanding of philosophy as correlated with poetry, with philosophy's influence on his outlook and its being a source of inspiration for him. The work revisited themes found in the earlier "Evanghelia naturii". A sympathizer of the National Liberal Party, with leading member Vintilă Brătianu a backer of his theories, Zeletin nevertheless joined the People's Party. He carried on a correspondence with several prominent intellectuals, including Iorga, Vasile Bogrea, Garabet Ibrăileanu, Gheorghe I. Brătianu, Nicolae Bagdasar, and his close friend Cezar Papacostea. He became gravely ill in 1930 and died four years later. He was buried in Bucharest's Sfânta Vineri Cemetery by his brother Dimitrie Motăș, aided by Papacostea and Bagdasar. He never married and had no children. A popular theorist of neoliberalism in the 1930s and 1940s, his ideas were shunned after the onset of the Communist regime in 1947. His works again began to see publication as part of anthologies in the 1970s, when his work on social development theory was re-evaluated. Following the 1989 fall of communism, his work reappeared in proper form and his ideas reentered debates about political ideology and national identity.