Tolkien's true life work, ultimately unfinished though it is THE SILMARILLION
In the Tolkien canon, THE SILMARILLION is the most highly contested of all his works. Constructed as a prehistoric history of the Universe, the book has the cultural significance of the Bible in Tolkien's universe. It is Tolkien's primary work, but it's also his most troublesome, in more ways than one. One thing you need to know. In Tolkien scholarship, there are two primary ways to refer to the "Silmarillion". One is the Silmarillion, the legendarium proper, and then the 1977 SILMARILLION, which may or may not be what Tolkien envisioned.
THE SILMARILLION, the book Tolkien spent all of his adult life writing, was, sadly, incomplete when Tolkien died at the age of eighty one in 1973. Naturally, this begs the question why did it take him decades to write the book, and it still be unfinished after all that time? Well, to understand that, you need to understand two things: the scope of the project, and how Tolkien worked.
The scope of the book was a complete imaginary history, a totally self-contained mythology, all written and developed for his home country, England (my home country as well). Imagine the Greek and Roman mythologies, all those myths and gods, developed by one man. Imagine Homer completely inventing all the gods for his stories. Imagine how hard that would be to come up with your own mythological traditions as such. No wonder Tolkien had such a hard time completing the work.
Now, the scope (which is extremely ambitious for any artist) was compounded by how Tolkien worked. First, he was a philologist first and foremost, and so before the stories he invented languages. All of these languages (which would have taken a life-time to develop on their own) had their own history, and are so interlocked with the mythology that you cannot remove them. He developed the main body of legends around these languages. Many features of the central body of legends changed relatively little over the years, but he wrote different versions of them at different times and in different styles. Some of the legends were set in poetry, those in annalistic histories, others in condensed summaries, and others in the more traditional (at least, for modern readers) novel format. A lot of these writings are also unfinished, due to Tolkien's perfectionist tendencies. Christopher Tolkien said that for most of his father's writing there existed a stable tradition from which Tolkien worked from, but there was no such thing as a stable text for the primary legends.
All this is tied to how Tolkien worked. C. S. Lewis famously stated that you did not influence Tolkien, you may as well as try to influence a bandersnatch. Tolkien would either take no notice of your criticism, or else he would start all over from the beginning. And so he did. A lot. Tolkien would reach a certain portion of the draft, be unsatisfied, and began the whole thing over again, while never reaching the end. Or Tolkien would have two copies of the same manuscript, one to be the fair copy and one to be working copy. Well, Tolkien would make conflicting revisions on both copies at separate times. How do you decide his final intent? Good
Wikipedia:
The Silmarillion is a collection of J. R. R. Tolkien's mythopoeic works, edited and published posthumously by his son Christopher Tolkien in 1977, with assistance from Guy Gavriel Kay,[1] who later became a noted fantasy writer. The Silmarillion, along with J. R. R. Tolkien's other works, forms an extensive, though incomplete, narrative that describes the universe of Ea in which are found the lands of Valinor, Beleriand, Numenor, and Middle-earth within which The Hobbit and The Lord of the Rings take place.
It explains why sauron became evil where/why the elves awoke.. the balrogs and dragons stc. I wish it was finished. Better than LOTR.