He is one of our greatest ever writers, a man who invented a world of characters and creatures which have become part of our collective folklore – a universe of hobbits, orcs, dwarves and trolls. Yet for all his global fame today, JRR Tolkien wasn’t always so popular.

In 1957 he sold the manuscripts of his greatest work Lord of the Rings for just £1,500 to an obscure university in the American Midwest.

“Tolkien was getting the cold shoulder at Oxford,” says Tolkien archivist and exhibition curator Catherine McIlwaine, who spent five years planning the show.

“He was gearing up for retirement and was supposed to be writing academic works on Beowulf and Middle English texts not fantastic stories about hobbits.

“He hadn’t made much money from The Lord of the Rings and was worried about his pension, so he sold them. It doesn’t seem much now but it was the equivalent of an academic salary at Oxford. By contrast, a first edition of The Hobbit inscribed by Tolkien went for £140,000 in 2015.”

The foresighted buyer was Marquette University in Milwaukee, Wisconsin – which has loaned the priceless manuscripts back to Oxford for the once in a lifetime exhibition, Tolkien: Maker of Middle-earth, which remains at the Weston Library until Sunday October 28.

The show – the greatest ever collection of materials related to the Hobbit creator, and so-called Inkling, features 200 items including never-before-seen illustrations, letters, drafts, fan mail and personal objects. Items cover the fruits of his literary imagination, such as his creation of the mythical Middle-earth in which his best known works are set, along with objects which shed light on his life as an artist, poet, medievalist, language scholar and family man.

While some items will remain in the Bodleian’s collection, on rotation in the library’s Treasures Gallery, others –including the manuscripts – will leave our shores, perhaps forever.

Catherine urged fans of the author’s works not to hang about, insisting there will not be another chance to see these items in the same place again in our lifetime.

The show has already proved immensely popular, attracting more than 1000,000 visitors since opening in June. It is the Bodleian’s most popular summer exhibition to date.

“It’s going to be sad when it goes,” says Catherine. “The last four months have been brilliant. Unfortunately the exhibition can’t stay any longer and the next one is waiting to go in.

“Marquette University have been very generous, though, and such an exhibition has never been done before. We have brought back The Hobbit and The Lord of the Rings and combined them with items from the Bodleian’s Tolkien archives. It has been special to bring everything together in one place.

“Oxford is the home of Tolkien, after all. This is where he lived and where he created his world.”

She said the popularity of the free exhibition – which will go on to New York then Paris – was testament to Tolkien’s immense appeal, only boosted by the Peter Jackson films of The Lord of the Rings and The Hobbit.

“His works sold well at first but not massively. Then in the mid 60s they took off in America. A pirate edition was published there due to a legal loophole which meant Tolkien received no royalties.”

The dispute became known as the War of Middle-earth. His reputation grew on university campuses.

“They are great stories full of adventure and plot twists,” says Catherine. “And because they are set in a far, distant time they don’t date in the same way as other author’s works. Each generation discovers it anew.

“Pretty much everything that came after Tolkien can be traced back to his work. There were fantasy authors before Tolkien but they are very much a niche. Now fantasy is one of the most popular genres and Tolkien is at the forefront of that.

“Some people only know the stories through the films,” she says. “While others have only read the books. There’s a circularity to it. So we thought it would be great to get his water colours, manuscripts and drawings. And we have so much beautiful stuff.”

Items also include academic lecture notes, maps, drafts of minor works, essays on fairy stories, research papers, letters, diaries and photographs. One of the most popular items is a 3D map which sits at the centre of the room.

They all serve to build a picture of Tolkien not only as a great writer and perfectionist but as a loving father – who wrote letters from Father Christmas to his children.

While many objects remain in the Bodleian’s collection, they are too fragile to stay on show. Catherine explains: “We can’t keep these items on display permanently. Even with low light levels they’d still get damaged.”

Catherine admits to have been taken aback by the exhibition’s popularity.

“We knew Tolkien was as popular today as ever but it’s been amazing to see the response to the exhibition,” she says. “It’s the first time we’ve had timed tickets – though it is still free, which is part of our remit to open up the collection to the public. We didn’t want huge queues though.

“We are an academic library and the collection is not available to the public so this is one way we can get people to come in and enjoy it.

“I’ll be sad to see it go. It’s going to feel empty.”

And Catherine’s favourite item? “That would be the last page from the Book of Mazarbal, which he wanted to go into The Fellowship of the Ring. It’s a record from the Dwarf Kingdom which he burned around the edges with his pipe and spotted with red-brown paint to make it look older. It’s written in runes and Elvish and is tattered around the edges.

“It’s like a real artefact of Middle-earth.”

Tolkien: Maker of Middle-earth is on at the Weston Library, Broad Street, Oxford until Sunday, October 28. Free admission but booking necessary at Tolkien.bodleian.ox.ac.uk

The show is accompanied by an essential, and beautifully illustrated book,Tolkien: Maker of Middle-earth, written by Catherine McIlwaine. It is priced at £40 hardback with a paperback edition available only at Bodleian Library Shops, priced £25.