The Forest History Society has helped to preserve, collect, and make available the history of forest and conservation history. This link brings you to their digital collections, but the entire website provides a lot of valuable information.
This collection consists of the correspondence of John Muir from 1856-1914. The vast majority of the letters were sent and received by Muir, although the collection also includes some correspondence of selected family members and colleagues. All of the letters include full-text transcriptions that can be searched as well as viewed alongside images of the originals. This collection is a part of a larger collection that is spread out across 35 institutions- most of the other collections are not digitized.