The Kenneth Spencer Research Library at the University of Kansas in Lawrence, Kansas holds a portion of the Critical Quarterly Archive (summary of contents). It was purchased in 1968 from Argosy Books in New York and contains: "584 Letters (including 9 retained copies of letters from CQ), Manuscript fair copies of poems, reviews, essays, and proofs." The abstract to the collections says:
Archives (letters received; some texts and proofs): of literary magazine. Submissions of material to Critical Quarterly; friendly personal news; literary gossip; academic life. Some polite notes from famous names. Some material concerning Cox or Dyson rather than the Critical Quarterly.As you might imagine, they contain Sylvia Plath archival materials... There are ten letters from Plath to the journal's editors A. E. Dyson and C. B. Cox from 1960 to 1961. I learned of the collection first through Linda Wagner-Martin's wonderful Sylvia Plath: A Literary Life (1999, 2nd ed. 2003).
I have withheld posting on this Sylvia Plath collection for many years as the ten letters formed an integral part of an essay I selfishly wanted to write on the poetry supplement Plath edited for the CQ in 1961 entitled American Poetry Now. I was able to write the essay, "'What's been happening in a lot of American poetry': Sylvia Plath as editor and reviewer", for the book I co-wrote with Gail Crowther, These Ghostly Archives: The Unearthing of Sylvia Plath (Fonthill), where it appears as Chapter 7. Another reason for not posting this was that the letters were to be printed in The Letters of Sylvia Plath, Volume 2.
This is one of those intricate archival stories where pieces are scattered and where sense can only be made once all those pieces are discovered, assembled, and studied. Papers relating to Plath's work on the American Poetry Now pamphlet are spread between at least five repositories: University of Kansas, Indiana University, Smith College, Emory University, and the University of Manchester. It is possible other materials are held elsewhere, too!
The correspondence at the University of Kansas is just part of the story. The letters are dated 16 January 1960; 22 February 1960; 1 January 1960 [1961]; 3 May 1961; 17 June 1961; 25 June 1961; 24 August 1961; 17 October 1961; 14 November 1961; and 12 December 1961. While it is evident that Plath received letters, it does not appear that she kept them (and the journal did not apparently retain carbons).
The initial letters from 1960 deal with publication of Plath's poetry. Namely her verse "Medallion" which was awarded a best poem prize as well as submissions. The eight 1961 letters all relate to American Poetry Now.
If you are interested in the other holdings outside of Kansas, you will find them in the following:
Emory University: Sylvia Plath: Subseries Prose
Indiana University: Plath mss
Smith College: Sylvia Plath Collection, Series XVI: Writings of Others Collected & Edited by SP
University of Manchester: Critical Quarterly Archive
All links accessed 20 April 2017 and 6 December 2018.