
The main correspondents in the file are Sylvia Plath (and Ted Hughes) and Elizabeth Ames, the Executive Director of Yaddo, though there are others. All material dates from 1959. Below is a list of letters.
1. Elizabeth Ames to Sylvia Plath and Ted Hughes, typed, undated, 1 p.
2. Sylvia Plath to Elizabeth Ames, typed, 11 February 1959, 1 p.
3. Elizabeth Ames to Sylvia Plath, typed, 13 February 1959, 1 p.
4. Sylvia Plath to Elizabeth Ames, typed, 22 February 1959, 1 p.
5. Elizabeth Ames to Philip Booth, typed, 25 Feburary 1959, 1 p.
6. Elizabeth Ames to John Sweeney, typed, 25 Feburary 1959, 1p.
7. Philip Booth to Elizabeth Ames, typed, 28 Feburary 1959, 2 p.
8. John Sweeney to Elizabeth Ames, handwritten, 6 March 1959, 2 p.
9. Elizabeth Ames to Sylvia Plath and Ted Hughes, typed, 3 April 1959, 1 p.
10. Sylvia Plath and Ted Hughes to Elizabeth Ames, typed, 8 April 1959, 1 p.
11. Elizabeth Ames to Sylvia Plath and Ted Hughes, typed, 12 August 1959, 1 p.
12. Sylvia Plath to Elizabeth Ames, typed, 24 August 1959, 1 p.
13. Sylvia Plath to Elizabeth Ames, typed, 5 September 1959, 1 p.
14. Elizabeth Ames to Sylvia Plath and Ted Hughes, typed, typed, 16 September 1959, 1 p.
In addition to the correspondence are miscellaneous items, all worth a mention. They hold two copies of Plath's application, which stretches to three pages and includes:
I. Name and address
II. Brief biographical note
III. Plans for work done at Yaddo
IV. Fellowships and awards
V. References (John Sweeney and Philip Booth)
VI. Preferred dates to visit Yaddo
They also have typed "file" notes by anonymously acronymed people - R. E., J. C. , and M. D. Z. R. E. I think is Richard Eberhardt. J. C. may be John Cheever. M. D. Z. I think is Morton Dauwen Zabel. At any rate, M. D. Z.'s comments on Plath are particularly interesting.
The New York Public Library is online here. The section of the library you'll want to contact is the Manuscript & Archives division. The finding aid to the Yaddo Records is online here. The Plath materials are in Box 276 (page 79 of the finding aid). To contact the archives, email mssref at nypl dot org.