Title of Tolkien book also appears on his gravestone

"It's the job that's never started as takes longest to finish," J.R.R. Tolkien once wrote.

While that's technically true, his newest book, "Beren and Lúthien," edited by his son Christopher, went on sale this week - about a century after he first conceived it.

Tolkien wrote volumes of unpublished materials set in his mythical Middle-earth of the "Lord of the Rings" series. Since his father's death, Christopher has been forming these writings into coherent books such as "The Silmarillion" and "The History of Middle-earth." The latest is likely the pair's swan song, as Christopher is 92 years old.

Like most of Tolkien's writing, the novel is a fantasy epic.

It follows two star-crossed lovers - the mortal human Beren and his immortal elven beau Lúthien. Her father Thingol, the Elven King of the forest land Doriath, disapproves of their romantic union, so he sends Beren on a quest that includes a faceoff with the evil creature Melkor. As young lovers are wont to do, Lúthien joins Beren anyway, and the two save each other time and time again through feats of bravery and love.

It might sound outlandish to a non-fantasy reader, but the story was deeply personal to Tolkien, and had been for much of his life.

For a clue to just how personal, just take a look at the gravestone in Oxford's Wolvercote Cemetery of John Ronald Reuel Tolkien and his wife Edith Mary Tolkien. Carved into the marble under their earthly names are Beren and Lúthien, respectively.

The idea for his new book came from his nearly lifelong relationship with Edith Bratt, as she was known before they wed.

When Tolkien was 16 years old, he fell in love with Bratt, a fellow orphan. His guardian, however, disapproved of their union. Eight years passed before he finally married her when he was 24 years old.

His eyes were still innocent that first year of marriage. Once, walking in the woods of East Yorkshire, Edith danced in a dell of white flowers, as he lovingly watched.

"Mr. Tolkien felt the kind of joy he must have felt at times he would never feel again," Tolkien biographer John Garth told the BBC.

Because the next year, Tolkien was deployed to the Battle of the Somme with the British Army, where he found himself in the trenches of the slaughterhouse of the First World War.

"Junior officers were being killed off, a dozen a minute," he later wrote. "Parting from my wife then . . . it was like a death," he later wrote.

Still, he was one of the lucky few. He caught trench fever and was sent home.

"When he came back from the trenches, with trench fever, he spent the winter convalescing," Garth said. "He'd lost two of his dearest friends on the Somme and you can imagine he must have been inside as much of a wreck as he was physically."

By then, the seed for these two characters who battle tremendous odds simply to hold each other, had been planted. A centerpiece scene of their story includes Lúthien dancing in a small valley of white flowers, as Beren lovingly watched.

Much of the story, in fact, was conceived long before he wrote "The Lord of the Rings" in 1954. Tolkien wrote a tremendous number of tales set in Middle-earth, most of them going unpublished. Many of the characters, in fact, existed long before he ever set pen to paper.

"It goes back a long way in my life, for it is my earliest actual recollection of some element in a story that was being told to me - not simply a remembered image on the scene of the story telling," Christopher wrote in the book's foreword. "My father told it to me, or parts of it, speaking it without any writing, in the early 1930s."

As book readers will soon find, he ended up writing many tales of the pair's adventures. What stuck with Tolkien the most, however, was that image of him lovingly watching Edith, of Beren watching Lúthien, dancing in a field.

Christopher wrote:

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In a letter to me on the subject of my mother, written in the year after her death, which was also the year before his own, he wrote of his overwhelming sense of bereavement, and of her wish to have Lúthien inscribed beneath her name on the grave. He returned in that letter … to the origin of the tale of Beren and Lúthien in a small woodland glade filled with hemlock flowers near Roos in Yorkshire, where she danced; and he said: 'But the story has gone crooked, and I am left, and I cannot plead before the inexorable Mandos.

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"Beren and Lúthien" may be the last Tolkien book the world receives that's guided by the editing hand of Christopher, who arguably knows how to navigate his father's unpublished writing better than anyone else.

It may well be the most fitting denouement.

"In my ninety-third year this is (presumptively) my last book in the long series of editions of my father's writings, very largely previously and is of a somewhat curious nature," he wrote. "This tale is chosen in memoriam because of its deeply-rooted presence in his own life, and his intense thought of the union of Lúthien, whom he called 'the greatest of the Eldar,' and of Beren the mortal man, of their fates, and of their second lives."