Seeking primary materials by and about Aurelia Plath, I've learned:
1) Sylvia Plath is said to have burned all her mother's letters to her ("upward of a thousand") in a bonfire in Devon in 1962. Only ten survive in the archives at Lilly Library at Indiana University and at Smith College. One interesting item is a Christmas card Sylvia kept in her purse, given by Ted to Warren and Maggie Plath at the time of Sylvia's funeral in 1963. Bridget Anna Lowe unearthed its story and wrote about it in Plath Profiles 5: Summer 2012.
2) There is no Aurelia Plath archive.
3) The JSTOR database lists no scholarly articles about Aurelia Plath. One unpublished thesis, "The influence of Aurelia Plath on Sylvia Plath: an interpretative biography," was written in 1977.
4) Aurelia kept her own journals, but they are not in any archive.
5) Aurelia's post-1977 letters are at Smith College; the ones actually sent to Ted Hughes are at Emory, although some drafts and carbons are at the Lilly Library.
6) Chief among Aurelia's primary materials is Letters Home, of course, but according to a review of Lessons from the Archive: Sylvia Plath and The Politics of Memory in Feminist Studies (Vol. 31, No. 3, 2005), "the full text of Aurelia Plath's intended introduction has not been published." That is true! Omitted is an anecdote about young Sylvia's sense of humor, and a passage saying the Plath children's questions about sex were always honestly answered and discussed. Lilly Library and Smith hold original and revised Letters Home typescripts.
7) Aurelia was interviewed by journalists and at least two filmmakers. At Washington University I found and copied an interview that was listed but had not been digitized: "Sylvia Plath's Letters Home: Some Reflections by Her Mother," by Robert Robertson in The Listener, Vol. 95, 1976, p. 515. In it Aurelia describes watching Sylvia build a bonfire and burning her second novel and "much else." This contradicts what The Other Ariel (page 57) says: "Aurelia Plath, visiting her daughter from June 21 to August 3, leaves no account of the incident immortalized by the poem ["Burning the Letters"].
8) Aurelia Plath's "Letter in the Actuality of Spring," in Ariel Ascending (1985), edited by Paul Alexander (pp. 214-217) is called an essay, but a footnote explains it's an excerpt from a letter. Alexander provided the title and received Aurelia's permission to print it as an essay.