Not much is known at this point, and Wayne Hammond and Christina Scull, unsurprisingly, collect the known facts in their blog post on the new book. John Garth has also produced an excellent blog post on the book, summarising, inter alia, what material Christopher Tolkien is likely to draw from.
When you've read these two blog posts, you probably will not need to read the rest of mine (except, perhaps, to be nice to me :-) ).
The book will start with the story in ‘its original form’, which Hammond & Scull presume will be the version published in The Book of Lost Tales 2 (though I will retain some small hope that it has proven possible to reconstruct some of the deleted pencil version under the published ink-version ...).
Then we will get ‘passages in prose and verse from later texts that illustrate the narrative as it changed.’ I assume – and very much hope – that this will be commented, and that Christopher Tolkien will take the opportunity to take a more ‘longitudinal’ view on the story, rather than the more ‘transversal’ view employed in The History of Middle-earth. By this, I mean the emphasis on the individual story and its evolution, contextualising the story amid earlier and later versions rather than amid the stage in the evolution of the legendarium to which it belongs.
Such an approach could possibly belong in the same scholarly tradition as Gergely Nagy's paper ‘The great chain of reading: (Inter-)textual relations and the technique of mythopoesis in the Túrin story’ in Jane Chance's (ed) Tolkien the Medievalist (Routledge 2003). Not (at least I would consider that quite unlikely) in the research points being made in the paper, but in the study of the variations of a single of the great stories.
And that is about it – nothing little more is known, really, though at least a little more may be said.
First, this book will not be anything like The Children of Húrin. I have seen a number of comments suggesting that some readers think that they will get something of that sort, but I am afraid that these are going to be disappointed. It seems likely that the book will be more accessible to the non-academic reader than The History of Middle-earth, but it will not be a self-contained narrative other than the promised full text of the original version, and personally I would be rather disappointed, if it is not accompanied by some notes and commentary by Christopher Tolkien on the evolution of the story.
Changes in style, form, and even changes in plot, show-case Tolkien's own changing ideas and opinions on literary aesthetics, ethics, etc., and these deserve to be contextualised by the person who is best able to do so. Here I am reminded of some Carl Hostetter's points in his paper, ‘Elvish as She is Spoke’, and I believe that just as Tolkien's changing linguistic ideas and tastes are reflected in the evolution of his invented languages, so are his changing literary ideas and tastes (among other things) reflected in the evolution of the great stories.
Whether there will be any new material seems at this point doubtful, but I will not give up hope entirely until I have the book in hand. John Garth has some intriguing speculations, and I would like nothing better than for this to be the case, but I have to admit that I doubt it, as news outlets claim that the texts for this book have been extracted from The History of Middle-earth to be presented in this new context, and they quote the HarperCollins press release as saying that the stories are here ‘presented together for the first time’.
Another possible source for new material might be in relation to the more personal aspect – for instance quotations from Tolkien's diaries from 1917, or from letters to his children about the 1917 incident or about the personal significance of the story. Such things would also be a dream come true.
Update 1 (2016-10-20):
The description at the HarperCollins product page (see below), is somewhat confusing. The description first mentions something “Painstakingly restored from Tolkien’s manuscripts and presented for the first time as a fully continuous and standalone story”, and later has the description of first presenting the story in “its original form” before moving on to present “passages in prose and verse from later texts that illustrate the narrative as it changed.” These two descriptions seem to me oddly inconsistent – certainly if the idea of the ‘original form’, as suggested by Wayne Hammond and Christina Scull, refers to the version given in The Book of Lost Tales 2.
Discussing this on Facebook, John Garth has suggested that he feels “all the more strongly that something derived from the texts behind the 1937 "Quenta Silmarillion" offers the best fit for the description – an "epic tale of Beren and Lúthien" that has been "painstakingly restored from Tolkien’s manuscripts and presented for the first time as a fully continuous and standalone story". And that does sound rather like Christopher's method in "The Children of Húrin", doesn't it?”
This seems a good guess, and it certainly fits the descriptions better than reproducing the Tale of Tinúviel from The Book of Lost Tales 2, though it would require a rather special interpretation of the word ‘original’ as used in the description. This would of course also make the connection to the centenary of the original event somewhat more tenuous (which I think would be a great shame), and, as John Garth also points out, it would make the choice of ‘passages in prose and verse from later texts’ rather less impressive, as the later texts will then be rather fewer (and with less significant changes).
I cannot help but hope for something less akin to the method from The Children of Húrin, as I find the story of Beren and Lúthien (or the Lay of Leithian) to be rather less suited for this treatment. Even if constrained to texts belonging to the creative period that brought us the 1937 ‘Quenta Silmarillion’, this approach will require a degree of conflation and homogenisation that is not, in my opinion, beneficial for our understanding of Tolkien and his own relationship with his mythology.
Instead, I will – at least for a while longer – keep hoping for a book where the central focus is on the earliest surviving version (the ‘original form’, in my eyes), and then highlighting and describing the author's changing literary thoughts, tastes and ideas through commented samples from the many later versions of the story. Regardless of any new material, that will still be a book, I would be very eager to include in my collection.
Links to a few articles (updated):
‘Beren and Lúthien’, Wayne G. Hammond and Christina Scull, blog post
‘Beren and Lúthien, a centenary publication’, John Garth, blog post (updated 2016-10-20)
‘Beren and Lúthien: Five Questions’, Nelson Goering, The LotR Plaza
‘Beren and Lúthien’, HarperCollins product page
‘Return to Middle-Earth for Houghton Mifflin Harcourt’, press release, Houghton Mifflin Harcourt
‘New Tolkien book: Beren and Lúthien’, Daniel Helen, The Tolkien Society
‘JRR Tolkien's Middle-earth love story to be published next year’, Alison Flood, The Guardian
‘New Tolkien tale to come from HC in 2017’, The Bookseller (staff)
‘J.R.R. Tolkien's Beren and Lúthien to be published in 2017’, Christian Holub, Entertainment Weekly
‘New JRR Tolkien book Beren and Lúthien to be published in 2017’, Tristram Fane Saunders, The Telegraph
‘New Tolkien Book Will Tell The Sweetest, Most Middle Earth Love Story Of All Time’, Claire Fallon, Huffington Post
‘HarperCollins is Publishing J.R.R. Tolkien’s Beren and Lúthien, 100 Years After It Was Written’, Maddy Myers, The Mary Sue