This news has been slowly inching its way across Tolkien circles for a couple of months now, having leaked out from publishers in a rather daftly irregular fashion, but it's official now, and I'll pass it along here in case anyone who hasn't heard is interested.
Tolkien's The Fall of Arthur will be published next year. It's an incomplete epic narrative alliterative poem (over 900 lines, plus outlines and drafts), in the same kind of modern English that he used for The Legend of Sigurd and Gudrun. He worked on it in the early 1930s, about the same time as The Hobbit. From what we know of it, it's probably based not so much on Malory as on other late medieval Arthurian literature.
If you want a sample, the opening lines are quoted in this news article, and the bits that appeared in Humphrey Carpenter's biography 35 years ago (so, you see, the existence of this piece is not news) are quoted (along with those opening lines again) by John D. Rateliff.
The mostly-unseen poem has not loomed large in Tolkien studies, though unlike the Sigurd material it's been generally part of people's conscious awareness. Some years later Tolkien wrote that one of the reasons he created his own mythology is that he felt that the Arthurian mythos, while straightforwardly British, was only "imperfectly" (his word) English; that is, he didn't feel it spoke that deeply to his own ethnicity, whatever the Welsh might feel about it. So it wasn't really his.
Still, that didn't prevent him at least starting to adapt it here. It will be interesting to see from the ancillary material (of which there must be a lot, as the new book is to be over 200 pages long) how long the poem might have been if finished. One eccentric Tolkien scholar built a whole argument about the importance of the Arthurian mythos to Tolkien out of a mistaken reading of the data that the poem is nine thousand lines long instead of nine hundred, and therefore one of Tolkien's biggest projects. Oops.