The documents, letters from the famous family of lighthouse builders and civil engineers, were found in an old tea chest, under a plank of wood, in the North Berwick Museum which closed down a decade a go.
The museum has stood empty for 10 years since it was closed by East Lothian Council in 2002 for failing to meet fire and disabled access regulations.
The documents, which included a letter from Robert Stevenson, the grandfather of Treasure Island writer Robert Louis Stevenson, who designed the Bell Rock Lighthouse, were found as workers surveyed the old building ahead of a planned £104,000 refurbishment.
Ian Goodall, of the Friends of the North Berwick Museum, said the discovery of the "treasure trove" of documents was an incredible find.
The documents were moved to the John Gray Archive Centre in Haddington, where they are now stored.