Letter up for auction in Sweden (and thus perhaps not showing up on everyone's radar):

Crafoord auctions

688 TOLKIEN, J. R. R. (1892-1973). TYPED & SIGNED LETTER. Typed & Signed letter to the Swedish biographer professor Arne Zettersten. 1 A 4-page sent from J.R.R. Tolkien, 76 Sandhurst Road, Headington, Oxford 19th May 1959 to Dr. Arne Zettersten in Lund Sweden. Signed "Yours sincerely J.R.R. Tolkien". They met as scientific linguists which ended in life long friendship. Zettersten published the biography J.R.R. Tolkien's Double Worlds and Creative Process: Language and Life, 2011. Tolkien writes to Zettersten as a linguist collegaue.