This comment got me to finally take my copy of Beren and Luthian off the shelf. The first 40 pages is all preface from Christopher and some necessary background detail on morgoth and elves of the first age. The remaining 200 pages seem to be the most complete form of beren and luthian possible made by stringing together excerpts of his father's writings that mention them into a complete narrative. many of the chapters seem to have a paragraph or two, usually at the beginning and end, with commentary from Christopher on why he chose to use that excerpt or where it came from. So not exactly a seemless novel but if you liked the story there is certainly more of it and some insights into tolkiens writing. Also the copy I have from a 3 books set called the great tales of middle earth has some pretty great illustrations as well

I believe it's more in depth, it may be like the other comment described the fall of gondolin book where it has multiple drafts and notes from Chris tolkien on his father's work

It really is the literary equivalent of math homework. There's so much to try to wrap your head around at first it's a terrible slog and I don't blame people for not enjoying it. But after putting in the work for awhile it just clicks and it's so satisfying seeing everything just sort of work.

There is a separate book that is a novelization of turins story called "Children of Hurin" that is an excellent more in depth telling of his story. I read that before reading the silmarillion and it made it so much more enjoyable having been introduced to characters and locations in an actual story. There are also novels for Beren and Luthian as well as The Fall of Gondolin. I believe Christopher tolkien put them together only a few years ago before he died