The Silmarillion (First Age) and Akallabeth (Second Age) are feigned history, somewhat like oral tradition. Tolkien was a leading expert on Beowulf.
They were written before the other books. No one would publish them; his son (and confidant) published them posthumously. (Even The Fellowship of the Ring was almost a vanity publishing by Houghton-Mifflin, family friends, who thought no one would read it, and the first printing was only 500 copies.)
They must be taken on their own merits, similar to old Norse Myth: separate stories that are loosely linked as an ongoing narrative.
I did not find them unreadable, and have read them a few times, although certainly they are not as enjoyable as a cohesive narrative like TLotR, which I have read more than two dozen times.
The Hobbit and The Lord of the Rings were based upon these books. Reading them expands upon many of the overt and covert references in them (especially in The Council of Elrond). There are many “Easter Eggs” to be found.
My favorite Easter Egg: Gimli the Dwarf offended the Silvan Elves - and Sindarin Prince Celeborn - by requesting a lock of Galadriel’s hair. The Dwarves and Elves were still nominally enemies, and Galadriel, a Noldorin Princess, was virtually a queen. But she saw he had a good heart, and she granted his request.
This takes on enormous resonance after reading The Silmarillion: Galadriel refused the same request three times from her cousin Feanor (who wanted to preserve her most beautiful hair in one of his creations), because she saw he did not have a good heart.
Feanor and Galadriel were both of the First Born in Valinor, and were the two most personally powerful Elves of all time.
I will get it and listen to it...I admire authors who can create an in depth world and make it come alive in their writing.