Amadís de Gaula (lost chapters of chivalric romance books; 14th century)
Amadís de Gaula is a landmark medieval romance novel series of the Iberian peninsula, and the lead character of the said novel. Garci Rodríguez de Montalvo, who wrote the 1508 edition of the books, admitted himself to have removed and altered segments from the originals. The three original unedited volumes of the Amadis saga are now lost, except for fragmentary remains in the Bancroft Library at the University of California, Berkley. 
Garci Rodríguez de Montalvo had in his possession three surviving volumes of the Amadis saga, which consisted of parchment scrolls bound into leather covers. He deleted many violent chapters and altered the original ending, in which Amadis is slain by his son Esplandian. Instead, the saga ends with a double wedding, in which both Amadis and his son marry their lady loves. This published version was enough of a success for Montalvo to write one final book in the saga.
- The Wikipedia page for Amadís de Gaula (aka Amadis the Gaul).
- Spanish Wikipedia source on the comments.