The Exmoor Society's collection of books, maps, letters and photographs used to be housed in two old storerooms but was disorganised and inaccessible.
The society began a two-year project to curate the material as part of its move to its new premises in Dulverton.
During the move from the Parish Rooms, a lost manuscript by Exmoor writer Hope Bourne was rediscovered after 45 years.
Archivist Dr Helen Blackman said other discoveries included several slide collections documenting changes in the moor's landscapes over 60 years, and material relating to the founding of Exmoor as a national park.
"All of this helps us gain a more complete picture of Exmoor's landscape, people and heritage," she added.
The archive, based at Dulverton High Street, will be officially opened later by Graeme Bonham-Carter.
His father, Victor Bonham-Carter, was the president of the society for more than 30 years and wrote several books and articles about Exmoor.
He was also a distant relative of the British actress, Helena Bonham-Carter.
An archive detailing the history of the fight to protect Exmoor as a national park has been opened to the public.