Spotting valuable books to raise thousands of pounds for Oxfam was an elementary task for volunteers who discovered a rare printing of a Sherlock Holmes story.

A Study in Scarlet, donated to a Harrogate branch of Oxfam, is being auctioned by Bonhams in Oxford next month - and could fetch £9,000.

Lot 67, valued at £7,000 to £9,000, is a Beeton's Christmas Annual containing the first printing of the first Sherlock Holmes story, which features illustrations by DH Friston and WMR Quick.

In the collector's item, the author introduces the detective to readers, in the story first issued in Samuel Beeton's Christmas Annual for 1887.

In March, Oxfam volunteer Andrew Chapman spotted a battered-looking copy of Graham Greene's 1931 novel, Rumour at Nightfall, in a box of donated books, and it raised £15,000 at auction - more than twice its guide price.

Mr Chapman said: "The Beeton annual is iconic because it is the first time the public are introduced to Sherlock Holmes - it's actually a beautiful book as well."

Rob McNeil, a spokesman for Cowley-based Oxfam, said: "It's quite astonishing such a magnificent array of valuable books are turning up in Oxfam bookshops, thanks to the eagle eyes of our volunteers.

"They are all very passionate about books and working as volunteers in our shops gives them the opportunity to raise money for the charity."

Simon Roberts, of Bonhams' books, maps and manuscript department, added: "It is extraordinary what emerges from these Oxfam shops.

"It's a snapshot of what people have read and collected over the past century.

"Some of these books have been handed in with little knowledge of their value."

Other Oxfam books going under the hammer include Memoirs of Sherlock Holmes, first edition, by Sidney Paget, inscribed 'To Mr R McKenzie. With best wishes 29th December, 1893'.