Gobber the Belch, and, in the film continuity, was the author of the first Dragon Manual. He apparently had a pet sheep named Willie, the last sheep of his unsuccessful flock, just as his descendant Gobber has his pet sheep, Phil.
Bork attempted many professions, but was always sabotaged by passing Dragons. Over the years, Bork accumulated a lot of knowledge about Dragons, writing the information down in a book. It was Bork who devised the Dragon Classes, and his work was passed down and modified through the many the Viking generations of Berk. His gathering of Dragon knowledge in spite of the risk to his own life merited his change of title.
It's also seen when Bork is writing the Dragon Manual, that he is left-handed.
His private archives of notes, thoughts, feelings, fears, and observations on dragons (and some recipes) was passed down within his family until inherited by Gobber. The notes were the resource Bork used to write the Book of Dragons.