#OTD in 1887.
L. L. Zamenhof published Unua Libro, the first publication to describe Esperanto, a constructed international language.
First published in Russian on July 26 [O.S. July 14] 1887, the publication of Unua Libro marks the formal beginning of the Esperanto movement.
https://en.wikipedia.org/wiki/Unua_LibroBooks in Esperanto at PG:
https://www.gutenberg.org/ebooks/bookshelf/34#books #literature