The Dua Libro de l' Lingvo Internacia (Second Book of the International Language) was the second publication to describe the international language Esperanto (then called Lingvo Internacia, "international language"), and the first book ever to appear entirely in Esperanto. It was published in Esperanto in Warsaw in 1888 by Dr. L.L. Zamenhof, shortly after the publication of his Unua Libro.
The Dua Libro was originally intended to be published in five or six volumes during 1888, with one volume appearing approximately every two months. Zamenhof's intention with the books was to provide some sample reading material in Esperanto, and to respond to questions about the language. During 1888 he would consider suggestions for changes to Esperanto, so that it would be fixed in its final form by the time of the publication of the final volume at the end of the year.
After the publication of the Dua Libro, Zamenhof decided that there was no need for the other planned volumes; instead, he published the Aldono al la Dua Libro de l' Lingvo Internacia (Supplement to the Second Book of the International Language). The only change he made in the language during 1888 was to change the spelling of the "time correlative" words (when, then, sometime, always, never) to their modern forms.
The Dua Libro consists of a foreword, two sections of comments on the Esperanto project, and 20 sections of sample Esperanto text. The sample texts include collections of model sentences, a translation of the Hans Christian Andersen story The Shadow, some popular sayings, and two poems.