A Literary Tour de France
Manuscripts
You can browse Robert Darnton's transcriptions for each of these booksellers by clicking the link to "Search letters pertaining to ..." link on each bookseller page.
Alternately, digital images of the original manuscripts are made available here as a reference to original source material. The original copies of these manuscripts are held at Bibliothèque Publique et Universitaire de Neuchâtel.
Warning, since these are images (not text) some of these files are quite large.