A swath of JRR Tolkien's original illustrations for The Hobbit are to be published for the first time this week as part of celebrations to mark the 75th anniversary of the book's publication.

The published version of The Hobbit includes around 20 illustrations by its author, as well as the well-known dust jacket painting of the mountains which Bilbo Baggins passes through on his adventures. But when HarperCollins began preparing for the book's 75th anniversary next year, the publisher discovered Tolkien had actually created more than 100 illustrations, which lay buried in his archive at the Bodleian Library in Oxford and were only recently digitised.

"That was a surprise. I thought there might be 40-50 in total," said publisher David Brawn. "But there are 110 Hobbit pictures, about two dozen of which haven't been published before."

Ranging from line drawings in ink to watercolours and sketches, the collected drawings will be published on 27 October as The Art of the Hobbit. HarperCollins hopes the collection and the anniversary will shed new light on the fantasy author – and on his first novel.

"It includes his conceptual sketches for the cover design, a couple of early versions of the maps and pages where he's experimenting with the runic forms, as well as a couple of manuscript pages," said Brawn. "It shows that Tolkien's creativity went beyond the writing, that it was a fully thought out conception. When he writes about the hobbit hole ["In a hole in the ground there lived a hobbit. Not a nasty, dirty, wet hole, filled with the ends of worms and an oozy smell, nor yet a dry, bare, sandy hole with nothing in it to sit down on or to eat: it was a hobbit-hole, and that means comfort"], he's designed it as well. And by doing that, it makes his description more vivid ... Tolkien was an accomplished amateur artist. He was a great admirer of Arthur Rackham and you can see a little bit of that style coming through."

Although The Hobbit was first published in September 1937, Tolkien delivered the manuscript to his publishers in October 1936, so HarperCollins feels justified in kicking off its anniversary celebrations early.

New editions of the novel itself are also lined up for publication this month. The publisher hopes to bring attention back to The Hobbit in advance of the much-anticipated release of Peter Jackson's film next year.

"People have celebrated Tolkien as a writer for years – you haven't been able to get away from him since all the books of the century polls," said Brawn. "But The Lord of the Rings has always been the focus since it was published in 1954 – it's a much more grown-up, significant book. It has overshadowed The Hobbit as a more old-fashioned, children's book, which has become known in the context of The Lord of the Rings. The anniversary allows us to move the spotlight back on to the book which started it all."