Sold by The Manhattan Rare Book Company:

Letter links:

Letter from J.R.R. Tolkien to Robert Burchfield • 11 June 1972 (#310)
Letter from Christopher Tolkien to Robert Burchfield • 27 June 1974 (#866)

Letters and books- including an autograph letter by J.R.R. Tolkien and two autograph letters and an autograph note by C.S. Lewis- from the collection of Robert W. Burchfield. [...]
Autograph letter signed by J.R.R. Tolkien to Burchfield dated June 11, 1972 and written from Merton College, apologizing for not returning the book (Kentish Place-Names) he borrowed. ... om/tolkien_burchfield.htm