The Fall of Arthur is set to be published this Spring by Harper Collins in the UK and Houghton Mifflin Harcourt in the USA. It is currently available for pre-order at Amazon.com.
From the book description at Amazon.com:
It is interesting to note that in the legend Arthur is taken to the isle of Avalon after his battle with Mordred, and in Tolkien's mythology the city of Avallónë is a harbor on Tol Eressëa, the Loney Isle, to where Frodo sails to recover from his hurts. Unlike Arthur, however, Frodo eventually died. The Arthur legend tells that he will return one day.Unhappily, The Fall of Arthur was one of several long narrative poems that he abandoned in that period. In this case he evidently began it in the earlier nineteen-thirties, and it was sufficiently advanced for him to send it to a very perceptive friend who read it with great enthusiasm at the end of 1934 and urgently pressed him ‘You simply must finish it!’ But in vain: he abandoned it, at some date unknown, though there is some evidence that it may have been in 1937, the year of the publication of The Hobbit and the first stirrings of The Lord of the Rings. Years later, in a letter of 1955, he said that ‘he hoped to finish a long poem on The Fall of Arthur’; but that day never came.