The Emmanuel d'Alzon Library at Assumption College in Worcester, Massachusetts, holds the letters Sylvia Plath wrote to Father Michael Carey between October 1962 and February 1963. Gail Crowther and I wrote about these letters a bit in our most recent "These Ghostly Archives" paper: " These Ghostly Archives 5: Reanimating the Past ". Some of the below was stated in this paper but this post should fill out the content of the letters... The letters are not "new". They were first published as an Appendix to Toni Saldívar's 1992 book Sylvia Plath: Confessing the Fictive Self (Peter Lang) on pages 201-206. However, at the time of the books publication Carey still held the letters, and not all of the letters were printed. To protect his interests, for example, Father Michael's name was redacted, and some of Plath's handwritten additions/postscripts were not included. Although the letters were published, I will give a brief paraphrase of each...
Sylvia Plath Info Blog by Peter K. Steinberg. The blog of A celebration, this is.