JRR Tolkien, once he returned from his Battle of the Somme, he made for us The Hobbit, and the Lord of the Rings, so he could have an excuse to make his own language, Elven, being a linguist. He has also written 23 other books about our Middle Earth. A new one is coming. Just to be clear, JRR died in 1973, and every book since then has been edited by his son, Christopher Tolkien.
The book is being published by HarperCollins, and will be released May 2017 The characters Beren and Lúthien have been in Fellowship of the Ring, the Silmarillion, and the History of Middle Earth series. It seems as though this will be a culmination of all the segments of the story, brought together into its own book.