The University of Reading holds some materials related to Sylvia Plath. From what I can find, the have some correspondence between Plath and members of the BBC, from 1960-1962. Additionally, they have two other collections relating to Linda Wagner-Martin's 1987 biography of Plath, published in the UK by Chatto & Windus Ltd. Both collections appear to be correspondence.
The first collection is Correspondence concerning Sylvia Plath, in three folders, a total of 391 items, from 1987-1989 (Call number CHATTO & WINDUS--CW 479/4). Here is the general contents:
Contents: correspondence concerning publication of Sylvia Plath: a biography / by Linda Wagner-Martin (1988) (ISBN 0701131268). Includes: 14 letters, a cv and bibliography, and a proposal for a book about Gertrude Stein and her family, from Wagner-Martin; 15 letters from literary agent and Plath's sister-in-law Olwyn Hughes; 4 letters and 3 telex messages from Simon & Schuster, publishers of the book in the United States of America; 8 letters from solicitors Simons Muirhead & Burton who read the book for libel; a legal opinion from Geoffrey Robertson Q.C. on copyright; 6 letters from literary agents Abner Stein; reviews of the US and UK editions; comments on the text from author, publishers, solicitors and Olwyn Hughes; and other papers.
The second collection has the same title, but contains 193 items in one folder, from 1985-1986 (Call Number CHATTO & WINDUS--CW 428/2). Here is the contents of this folder:
Contents: correspondence in preparation to publish Sylvia Plath: a biography / by Linda Wagner-Martin (1988). Includes: 6 letters and an author's questionnaire from Wagner; 12 letters from Simon & Schuster, publishers of the book in the United States of America; 1 letter and a detailed list of comments on the ms from Ted Hughes; 10 letters from literary agents Olwyn Hughes; 4 from literary agents Abner Stein; and other papers.
The first collection is Correspondence concerning Sylvia Plath, in three folders, a total of 391 items, from 1987-1989 (Call number CHATTO & WINDUS--CW 479/4). Here is the general contents:
Contents: correspondence concerning publication of Sylvia Plath: a biography / by Linda Wagner-Martin (1988) (ISBN 0701131268). Includes: 14 letters, a cv and bibliography, and a proposal for a book about Gertrude Stein and her family, from Wagner-Martin; 15 letters from literary agent and Plath's sister-in-law Olwyn Hughes; 4 letters and 3 telex messages from Simon & Schuster, publishers of the book in the United States of America; 8 letters from solicitors Simons Muirhead & Burton who read the book for libel; a legal opinion from Geoffrey Robertson Q.C. on copyright; 6 letters from literary agents Abner Stein; reviews of the US and UK editions; comments on the text from author, publishers, solicitors and Olwyn Hughes; and other papers.
The second collection has the same title, but contains 193 items in one folder, from 1985-1986 (Call Number CHATTO & WINDUS--CW 428/2). Here is the contents of this folder:
Contents: correspondence in preparation to publish Sylvia Plath: a biography / by Linda Wagner-Martin (1988). Includes: 6 letters and an author's questionnaire from Wagner; 12 letters from Simon & Schuster, publishers of the book in the United States of America; 1 letter and a detailed list of comments on the ms from Ted Hughes; 10 letters from literary agents Olwyn Hughes; 4 from literary agents Abner Stein; and other papers.
The Library at University of Reading web site is here.