The New York Public Library holds the Yaddo Records, 1870-1980. The finding aid to the collection is online here.
Materials relating to Plath and Hughes may be found in the following series:
Series V: Yaddo Corporation Records, 1926-1980
- A. Guest Files
- 2. 1940-1976
- Box 276, Folder 2
I have a request out to the NYPL for more information as to the contents of Box 276, Folder 2. Will report back when I hear something!