|9 Princess Street, London NW1|
The letter is brief, just two paragraphs of one sentence each and is signed under her married name Sylvia Hughes. The letter politely cancels plans to meet on Saturday night in part because it was Plath's turn to work at the office (The Bookseller) and hopes they can arrange to meet again sometime in the future. The letter was sent to the Taylor's at 25 Kensington Gate, London (map).
If you search the Taylor collections at Vanderbilt, you will also see another finding aid for a different collection of his papers: The Peter Taylor Papers (MSS. 591). This collection, too, has a hit for Plath in Series 1: Correspondence, Incoming Correspondence, Box 6 (O-Q), Folder 8: Pierce - Plath. However, this is the wrong Plath! This one is from James Plath and is dated April 17, 1990. Its subject is the new independent arts journal, Clockwatch Review.
Thanks to Molly Dohrmann of Special Collections and University Archives Vanderbilt University for her assistance.
Eleanor Ross Taylor reviewed Ariel in her article "Sylvia Plath's Last Poems" in January 1967 issue of Poetry (pages 260-262). A couple of years back, some books (maybe all?) from the Taylor's library were sold via Between the Covers Rare Books. Among those books was Eleanor Ross Taylor's Ariel, with her ownership signature on the front free end paper, which I received as a gift from a friend.
All links accessed 3 June 2014 and 27 January 2015.