Fundamento de Esperanto

Fundamento de Esperanto (English: "Foundation of Esperanto") is a 1905 book by L. L. Zamenhof, in which the author explains the basic grammar rules and vocabulary that constitute the basis of the constructed language Esperanto. On August 9, 1905, it was made the only obligatory authority over the language by the Declaration of Boulogne at the first World Esperanto Congress. Much of the content of the book is a reproduction of content from Zamenhof's earlier works, particularly "Unua Libro".
Content.
"Fundamento de Esperanto" consists of four parts: a foreword, a grammar section, a collection of exercises, and a dictionary. With the exception of the foreword, almost everything in the "Fundamento" comes directly from Zamenhof's earlier works, primarily "Unua Libro". Esperanto, however, underwent a minor change in 1888 in "Aldono al la Dua Libro", in which Zamenhof changed the ending of the temporal correlatives ("when", "then", "always", "sometimes", "never") from "-ian" to "-iam", so the Esperanto of the "Fundamento" is slightly different than that of "Unua Libro".
The grammar and dictionary sections of the "Fundamento" are in five national languages: French, English, German, Russian, and Polish.
Declaration of Boulogne.
"Fundamento de Esperanto" was made the official source of Esperanto in the fourth article of the Declaration of Boulogne at the first World Esperanto Congress in Boulogne-sur-Mer, France:
Equal to the "Fundamento" are the "Oficialaj Aldonoj" ("Official Additions"). To date, there have been nine Official Additions. The foreword of the "Fundamento" states:
That authoritative institution is the Akademio de Esperanto.