Dua Libro de l' Lingvo Internacia (English: Second Book of the International Language) was the second publication to describe Esperanto, then called the International Language (Esperanto: Lingvo Internacia), and the first book ever to appear entirely in Esperanto. It was published in Warsaw in 1888 by L. L. Zamenhof, shortly after the publication of Unua Libro.
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 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.
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.