A donated first edition book on ballet featuring a lithograph by Pablo Picasso and a design by Matisse could raise more than £500 for Oxfam Cymru.
A Picasso lithograph of a dancer features in Le Ballet
Charity worker Marilyn Willis was sorting through contributions at Bangor's Oxfam shop when she found the copy of Le Ballet by Boris Kochno.
The book will now be sold in the charity's Valued in Wales auction in Cardiff in July.
Mrs Willis said: "It was a very special book - you could not miss it."
The copy of Le Ballet, which was compiled in the 1950s, is covered in red cloth and is still kept in the original acetate and cardboard cover.
The Picasso lithograph depicts a dancer and the book also features photographs of Nijinsky and set designs by Parisian artist Toulouse-Lautrec.
WHAT IS A LITHOGRAPH?
It can be used to print text or artwork onto paper
It works because of the repulsion of oil and water
Modern high-volume lithography is used to produce posters, books and credit cards
In the 1890s colour lithography became popular with French artists including Toulouse-Lautrec
Mrs Willis said it was obvious the book, which came from a mystery donor, was out of the ordinary because it stood out so much from the works she usually sifts through.
"It's a startling book and very beautiful," she said.
"I always refer to an American website whenever we have something in that I think might be worth more than £50, and this one seemed to changing hands for up to £500."
Mrs Willis and other staff from the charity shop have been on specialist courses to help them recognise donations that could make the charity money.