Shoot Me With Flowers, printed in Guyana in 1974, contains the first poems published by John Agard, whose work is on the GCSE syllabus.
A copy of the little book turned up at the Oxfam shop on Boroughbridge High Street and was spotted by staff.
Shop manager Mel Wrightson said: “Jon Purday, our book-sorting volunteer, checked out the book and found that only a few copies ever got to England, and that not even the British Library, which gets a copy of everything that’s published here, had one.
“We got in touch with the library and they were delighted to buy it from us.”
Nicole-Rachelle Moore, the Curator for Caribbean Collections at the British Library was delighted with the find.
“John Agard is such a popular and important British writer and a Fellow of the Royal Society of Literature," she said. “We have all his other work in the national collection but not this one, because such a small number were privately printed.
“It’s not so much that it’s hugely valuable – copies have changed hands online for around £20 – but that it’s a rare little treasure that completes our collection.”
Mel added: “It just goes to show what special and interesting things come into Oxfam Boroughbridge.
“And the fact that John Agard’s best-known poem on the school syllabus is Checking Out Me History and we found his first book in Black History Month is a coincidence that made me smile.”
John Agard came to Britain shortly after writing Shoot Me With Flowers.
Since then he has published over 50 books of plays and poems, won the Queen’s Gold Medal for Poetry, and toured Britain and the world performing his work.