The saga of Day's attempt to publish The Hobbit Companion, his guide to the works of J R R Tolkien, has as many twists and turns as a work of fiction, with accusations of literary theft, hobbit heresy and threats of an injunction all being used to stop the book.
Day's troubles began when he submitted his manuscript to his original publisher, HarperCollins, which also publishes J R R Tolkien's books and showed it to the Tolkien estate out of courtesy.
The estate is highly protective of the works of Tolkien, an Oxford University philologist who started writing his stories of Middle Earth when he became bored with writing examination scripts.
When the estate read Day's manuscript it demanded that HarperCollins abandon the project and threatened the author with legal action.
The estate accused Day of merely replicating Tolkien's invention and defaming the hobbit character, rather than critically appraising it.
According to Day, however, his book was "a sort of everything you always wanted to know about hobbits but were afraid to ask".
He was astonished by the response to his volume, which he had been writing for 12 months, particularly when HarperCollins decided not to go ahead.
"I'm utterly baffled by the venom of the estate over a book which is utterly complimentary toward J R R Tolkien," he said. "How anyone can take offence at such a gentle, beautifully illustrated book will be a total mystery to anyone who sees it."
Day sought legal advice, found a new publisher, and began rewriting the book. Earlier this year Pavilion Books agreed to publish - only to receive a prompt letter from the estate threatening an injunction.
Day and Pavilion decided to send a copy of the reworked manuscript to the estate, which finally agreed it was acceptable.
HarperCollins project director David Brawn explained that the estate was willing for the book to go ahead now because it was more about Tolkien's use of words than a rewriting of the plot.
"It's now far enough divorced from the fictional world of the hobbit not to be a copyright issue. It's much more about the linguistics," he said.
Pavilion is so convinced that the book will appeal to a mass audience that it is printing 100,000. "To suggest that it has minority, obsessional interest is to misunderstand what it's about," said publisher Colin Webb.
Tolkien's books certainly continue to have wide appeal. Since The Hobbit was first published in 1954 they have continued to be among the nation's favourite tales: earlier this year, The Lord of the Rings came top in a Waterstone's survey of best books of the century.