So, it's now been officially announced that the next book by Tolkien (that is, the next book of new material by JRRT) will be out in June:
THE NATURE OF MIDDLE-EARTH, edited by Carl Hostetter (co-editor of TOLKIEN'S LEGENDARIUM).
The write-up on Amazon doesn't have much information yet, other than that the book is 400 pages and the official release date June 1st.
I gather that this book draws from the period when Tolkien had largely abandoned or set aside work on his various narratives and was shifting more and more into world-building. Or to put it another way, rather than a grand narrative here we'll be seeing Tolkien's attempt to set down as much as he cd about Middle-earth.
I suspect it'll feel rather like LETTERS, where he addressed so many queries from his readers. For those of us who came along too late to write to the Professor and pose questions ourselves, this book just might contain the answers to things we've always wanted to know.
In short, not a book for the casual fan, but it promises to be full of good things for those of us who want to know all we can about Tolkien's subcreated world. Congratulations to Carl for bringing all this disparate material together.
I've already pre-ordered my copy.
P.S.: Here's the announcement of the news in THE GUARDIAN: