The Silmarillion is J.R.R. Tolkien’s first and last book.
Professor Tolkien’s notes first dated to 1917 as he began developing the characters, stories and ideas for what was to become a Middle-earth mythology while recovering in a WWI military hospital from trench fever.
With the success of The Hobbit (published 40 years before The Silmarillion), Tolkien’s Sil was rejected by the publishers who requested more stories about hobbits. This resulted in 3 installments of The Lord of the Rings 17 years later between 1954 and 1955. But he never gave up on his expansive work on this inspired world.
Sadly, J.R.R. Tolkien passed away in 1973 before completing The Silmarillion. But through the devotion and efforts of his son Christopher, Professor Tolkien’s work was finished and published on September 15, 1977.
“The Silmarillion is the history of the War of the Exiled Elves against the Enemy, which all takes place in the North-west of the world (Middle-earth). Several tales of victory and tragedy are caught up in it; but it ends
with catastrophe, and the passing of the Ancient World” ― J.R.R. Tolkien in Letter 131 to Milton Waldman