Ever wondered where those nowadays-ubiquitous Keep Calm and Carry On posters first came from? Nope, me neither – and frankly, more fool both of us. Someone posted the link to a video purporting to tell the "story of how Barter Books found the Keep Calm and Carry On poster & made it a global hit" on Twitter the other day, and honestly, I wouldn't have clicked on it had I not been intrigued by the fact that it featured the name of one my favourite bookshops. But click on it I did, and here's what I discovered: it turns out that not only are Barter Books' owners Stuart and Mary Manley brilliant at selling books, they're also brilliant at giving birth to country-wide trends, too.

The video tells the story of the Keep Calm posters, which were commissioned by the government during the second world war as part of a wider poster campaign designed to boost morale among the civilian population. Some 2.5m copies of our poster were printed, but in the end they were kept back; "held in reserve, intended for use only in times of crisis or invasion", which happily never came. 50 years later, Stuart found one in a box of books he'd bought at auction, and Mary put it up by the till. Apparently, customers were so taken with it that the pair began making copies - and an iconic noughties image was born.

It's a lovely video, as much for its shots of Barter Books - once a Victorian railway station; now overflowing with well-stocked shelves - as for the story it tells. But the story's a fine one, too, and the sentiment of the poster, which overexposure had led me to dismiss as trite, becomes moving and inspiring again when resituated in its original context of genuine threat and principled resistance. Enjoy.