L. L. Zamenhof
Dua Libro de l' Lingvo Internacia ("Second Book of the International Language"), usually referred to simply as Dua Libro, is an 1888 book by L. L. Zamenhof. It is the second book in which Zamenhof wrote about the constructed language Esperanto, following "Unua Libro" in 1887, and the first book to be written entirely in the language.