As Dmitri Gorbatov has pointed out in the guest book at myaskovsky.ru, the correspondence between Sergei Prokofiev and Nikolai Myaskovsky was published in Russian in 1977 as a volume of some 700 pages, under the editorship of Dmitry Kabalevsky. Although a selected correspondence of Prokofiev appeared in 1998, edited and translated by Harlow Robinson, it contains only a few of Prokofiev’s letters to Myaskovsky, and none by Myaskovsky himself.
It would indeed be useful to have the 1977 volume in English translation. But as Mr Gorbatov suggests, such a project is unlikely to awaken the interest of English and American publishers any time soon. While the music and biography of Shostakovich appear to be enjoying something of a publicity “boom”, if such a word can be used in such a context, other 20th century Russian composers are suffering from an undeserved neglect.
Incidentally, the Russian text of the Myaskovsky-Prokofiev correspondence has been scanned and uploaded to this URL.