It has come to light on the Tolkien Collector’s Guide that Harper Collins and Houghton Mifflin have plans to publish a new book titled The Nature of Middle-earth in 2021 containing previously unpublished writings by J.R.R. Tolkien.
The book has been edited by Tolkien expert Carl F. Hostetter who heads the Elvish Linguistic Fellowship. The materials on which the book is based were sent to Hostetter in photocopy by Christopher Tolkien, before his passing, for potential publication.
The details were first released in a catalogue for the Frankfurt Book Fair in 2019. The description, excerpted from the catalogue (page 64), follows.
The first ever publication of J. R. R. Tolkien’s final writings on Middle-earth, covering a wide-range of subjects, and the perfect next read for those who have enjoyed Unfinished Tales and the History of Middle-earth series and are hungry for more.Harper Collins Publishers, Frankfurt BookFair 2019, Fiction Translation Rights
The Nature of Middle-earth will comprise numerous late (c. 1959-73) and previously unpublished writings by J.R.R. Tolkien on the “nature” of Middle-earth, in both chief senses of that word: both metaphysical and natural/historical.
For Tolkien fans, readers, and scholars interested in learning more about Tolkien’s own views on Middle-earth. It will appeal in particular to those readers who enjoyed Unfinished Tales, and some of the later volumes of the History of Middle-earth. Indeed, many of the texts to be included are closely associated with materials published in those places, and were sent to Hostetter, specifically, in photocopy by Christopher Tolkien for potential publication.
Much as Unfinished Tales forms an unofficial thirteenth volume of the History of Middle-earth, this new book will sit very nicely alongside as an unofficial 14th volume. Of particular note, given the impending Amazon series, are several texts detailing the lands, flora, and fauna of Númenor, and the lives of Númenóreans.