Hippocampus is now listing, on the “New Books” page, H. P. Lovecraft: Letters to E. Hoffmann Price and Richard F. Searight. Seemingly shipping soon.
Also, my copy of the Lovecraft Annual 2020 had arrived, and a filler paragraph informs me of a new book of letters. I was aware of Eccentric, Impractical Devils: The Letters of August Derleth and Clark Ashton Smith, which appeared for Halloween 2020. I was not aware of its planned companion volume, Born under Saturn: The Letters of Clark Ashton Smith and Samuel Loveman. This is possibly because the book has not yet appeared, though the Annual anticipated it appearing in 2020.
Letters to EHP & RFS will be out in May, as stated on the first page of the site. IIRC, Born under Saturn was postponed so it could be paired with the expanded second edition of Loveman’s Out of the Immortal Night — no release date for this pair yet, but I hope this year.