JRR Tolkien's The Fall of Gondolin, which the author described as "the first real story" set in Middle-earth, is to be published as a stand-alone book for the first time.
The book charts the story of an elven city sacked by the Dark Lord, Morgoth.
The author started writing it in 1917, before returning to Middle-earth for The Hobbit and The Lord of the Rings.
Tolkien Society chair Shaun Gunner said many fans regarded The Fall of Gondolin as "the Holy Grail of Tolkien texts".
The Fall of Gondolin is the second "new" work to be edited and released by Tolkien's son Christopher Tolkien in two years.
It has come as a surprise to many, since Christopher - who is 93 - described the 2017 release of Beren and Luthien as "(presumptively) my last book in the long series of editions of my father's writings".
The only Middle-earth-based novels to be published by JRR Tolkien in his lifetime were The Hobbit and The Lord of the Rings trilogy. He died in 1973.
Christopher, once referred to by his father as his "chief critic and collaborator", has since spent much of his life organising his father's unpublished writings.
He has edited and posthumously published works including The Silmarillion and The Children of Hurin.
It's thought Tolkien began writing The Fall of Gondolin while recovering from active service during World War One in 1917.
The story will act as a missing link, chronicling the ancient history of Middle-earth before the events in The Lord of the Rings.
Responding to the news, Mr Gunner said: "We never dared to dream that we would see this published.
"The Fall of Gondolin is, to many in the Tolkien community, the Holy Grail of Tolkien texts as one of Tolkien's three Great Tales alongside The Children of Hurin and Beren and Luthien."
The novel, set to arrive in bookshops on 30 August, is illustrated by Alan Lee, whose work on books The Lord of the Rings and The Hobbit provided the inspiration for Peter Jackson's film adaptations.
Other artists to be published posthumously
Much of Tolkien's body of work has been published since the author's death. Here are a few other writers and artists who have had more work published following their death than during their lifetime...
Few of Kafka's works were published during his lifetime - although one of his most famous pieces, The Metamorphosis, was, and received little attention.
Upon his death in 1923, he ordered his manuscripts to be destroyed. Fortunately, these instructions were ignored and novels including The Trial and The Castle were published - leading Kafka to become one of the 20th Century's greatest literary figures.
He now even has an adjective derived from his name - "Kafkaesque" means to have nightmarish or illogical qualities.
Larsson was a renowned journalist and editor in Sweden before his death from a sudden heart attack in 2004.
His best-sellers The Girl with the Dragon Tattoo and its two sequels were published posthumously and subsequently adapted into films.
Vincent van Gogh
The post-impressionist artist was known among other artists, but only sold one painting during his lifetime.
Following his death, he has become one of the most famous and sought-after artists in the world.
The singer released one studio album during his lifetime, Grace, which received good reviews upon its 1994 release.
His second album and numerous compilations have been released since his drowning in 1997.