A new blog post from S. T. Joshi gives the title of… “the next volume in the Lovecraft letters series”. It’s to be Letters to Hyman Bradofsky and Others. The book’s text is evidently still in the process of being assembled and edited, though, at present.
Bradofsky was an amateur journalist active in the NAPA, a collector of amateur journalism, and editor of his accomplished amateur journal The Californian.
After that book of letters should come the two-volumes of Long letters, and then the mega-index.