The 800-page Dawnward Spire, Lonely Hill: The Letters of H. P. Lovecraft and Clark Ashton Smith is out, albeit at first only in a limited-edition hardback. Apparently released summer 2017, though Amazon UK knows nothing of it.

Other volumes in recent years are: Letters to C. L. Moore and Others (2017); Letters to F. Lee Baldwin, Duane W. Rimel, and Nils Frome (2016); Letters to J. Vernon Shea, Carl F. Strauch, and Lee McBride White (2016); Letters to Robert Bloch and Others (2015); and Letters to Elizabeth Toldridge and Anne Tillery Renshaw (2014). Paperback only, as yet.