HarperCollins is going to be publishing Tolkien's Children of Húrin as a stand-alone volume next year. According to the press release (which I haven't been able to find on line), the text was created by Christopher Tolkien's painstaking editing together of Tolkien's many drafts. The book will include a new map by Christopher Tolkien and a jacket and color paintings by Alan Lee.
Quote from Christopher Tolkien:
It has seemed to me for a long time that there was a good case for presenting my father's long version of the legend of the Children of Húrin as an independent work, between its own covers, with a minimum of editorial presence, and above all in continuous narrative without gaps or interruptions, if this could be done without distortion or invention, despite the unfinished state in which he left some parts of it.
It is not clear from the press release (and I have absolutely no insider knowledge) that there will be anything that was previously unreleased in the book.
Various different versions of the tale of the children of Húrin have previously been published:
1977 in The Silmarillion as "Of Túrin Turambar" (prose).
1980 in Unfinished Tales as "Narn i Hîn Húrin" (prose).
1984 in The Book of Lost Tales, Part II as "Turambar and the Foalókë," and "The Nauglafring," (prose).
1985 in The Lays of Beleriand as "The Lay of the Children of Húrin" (verse in alliterative long-lines).
1994 in The War of the Jewels as "The Wanderings of Húrin" (prose).
From the press release, it seems as if these variants will be stitched into a coherent whole in the same the way that Christopher Tolkien brought together disparate texts to create the 1977 The Silmarillion
So, The Children of Húrin will not be a "new" book, but I think its release as a stand-alone volume is a very good thing for a variety of reasons.
First, the material is powerful and evocative and goes back to the very beginning of Tolkien's writings about Middle-earth, as it was originally inspired by Tolkien's reading of the Kullervo cycle in the Finnish Kalevala. The Túrin story is the element of Tolkien's legendarium that is the most "novelistic" in form, with more dialogue and detailed action than the more sweeping, historical style of the published Silmarillion. But it has been very difficult for most general readers to get a handle on the story because of the way Christopher Tolkien had to edit and publish the texts: they were part of scholarly editions, designed in large part to provide a documentary record of J.R.R. Tolkien's work. As such, they are very difficult simply to read for pleasure, the way we read The Lord of the Rings; Christopher Tolkien had to present texts, then explain variants, gaps and contradictions. So reading any of the post-Unfinished Tales pieces is a very difficult exercise for people who do not have a lot of experience with these sorts of editions (i.e., nearly anyone who is not a medievalist).
Second, by compiling everything into a whole, Christopher Tolkien is doing exactly what his father eventually envisioned for The Silmarillion (at least as best we can tell from the published record). It was not supposed to be a "novel" like The Lord of the Rings but was instead the volumes of Translations from the Elvish by B.B. created by Bilbo in Rivendell from his translations of various books of lore. Thus the Silmarillion (the legendarium, as distinct from The Silmarillion, the 1977 text), was conceived of as a tapestry woven from materials taken from various other texts, some poetry, some prose, some fragmentary, some contradictory. Although possibly frustrating to general readers who want to get to the story, the incredibly complex layering that is generated by such an approach is what gives all of Tolkien's work the impression of immense depth (the "vast backcloths" to use Tolkien's own description). Gergely Nagy, in what I think is the best article written on Tolkien in the past decade, talks about the "Great Chain of Reading" that links together various authors, compiliers, historians and translators (Heorrenda, Pengolad, Bilbo, Frodo and Sam, etc.). The Children of Húrin should give us another example of the final effects of that Great Chain.
Perhaps as a result of the enormously unfair criticism leveled at him after the 1977 publication of The Silmarillion, Christopher Tolkien went more in a scholarly direction rather than continuing to be a synthesist and compiler. All of his work in The History of Middle-earth is incredibly valuable, but it was not the only possible approach. If The Children of Húrin volume is more like The Silmarillion, then it will be a return to something that Christopher Tolkien himself does very, very well and is perfectly in keeping with the underlying conception of the legendarium.
So even though in one sense I have 'already read' the new book, I am definitely looking forward to its release in April 2007 and will certainly enjoy reading it straight through in a way I have not previously been able to do with the Túrin materials.