When he sat down to draw a boy talking to a horse, the illustrator Charlie Mackesy was working out his own feelings. But his drawing of a horse confessing the bravest thing he’s ever said was “Help” became an online sensation. The book that image inspired is now topping charts on both sides of the Atlantic, with Mackesy’s publisher printing hundreds of thousands of copies to meet demand.

Mackesy, who has been a cartoonist for the Spectator and a book illustrator for Oxford University Press, says the straightforward, heartfelt conversations between the characters in The Boy, the Mole, the Fox and the Horse were drawn from “conversations I’ve had with my friends about what life really means, what’s important; it was a way for me to think aloud on paper with words and drawings.”

‘What is the bravest thing you’ve ever said?’ asked the boy. ‘Help,’ said the horse. Illustration: Extracted from The Boy, the Mole, the Fox and the Horse by Charlie Mackesy/Ebury Press

The first, featuring the horse and the boy, stemmed from “a conversation I had with my friend Bear [Grylls] about what courage really looked like” and “the bravest thing we’d ever done”. While Grylls may be “an emblem of courage”, Mackesy continued, “the bravest thing I’d ever done was when I was struggling and had the courage to ask for help. So I drew it.”

“I put that up on Instagram and forgot about it, and the next thing I knew was that hospitals and institutions had been using it, and the army had been using it for PTSD, it went crazy. I wasn’t aware of it. Occasionally I’d get emails saying ‘I hope you don’t mind we used it in our therapy unit, it’s helping people realise it’s a brave thing to show weakness’.”

Mackesy made the drawing at “a time of life when I had lost a friend, when things make you think harder about what really matters”.

“All four characters represent different parts of the same person,” he explained, “the inquisitive boy, the mole who’s enthusiastic but a bit greedy, the fox who’s been hurt so is withdrawn from life, slow to trust but wants to be part of things, and the horse who’s the wisest bit, the deepest part of you, the soul.”

An editor at Ebury, Laura Higginson, came across Mackesy’s pictures last summer, when the illustrator had already amassed 30,000 followers on Instagram. “It wasn’t a formal proposal from a literary agent,” she said, “I saw it and I thought ‘this is really moving’.” Higginson met him at an exhibition of his illustrations but according to the author, when he came up with the narrative thread which links his characters together “we both started crying”.

The writer and illustrator Charlie Mackesy. Photograph: David Loftus

Published on 10 October, the book has sold 51,610 copies according to BookScan. It’s No 1 on Amazon, has hit bestseller charts in the UK and the US, and has been shortlisted for the Waterstones book of the year. “We initially costed the book on 10,000 copies,” said Ebury managing director, Joel Rickett. “Two weeks after publication we have 200,000 copies in print and we’re scrambling to find the capacity to print another 100,000.”

According to Waterstones’ Kate McHale, the hardback will be a key Christmas title. “It’s doing exceptionally well,” she said. “We’ve been absolutely delighted – it’s such a lovely book, about having courage in difficult times, about love and friendship and how that builds resilience, which makes it universal. People are coming in and buying multiple copies, for themselves and for their friends.”

Higginson said that readers – including one woman who bought 25 copies at an Oxford signing – were responding to Mackesy’s ability to “put just the right words and images together, as well as the universal values in the book”.

“No matter who you are, what age you are, what your belief system is, there are universal feelings and experiences which unite us all,” she said. “That might sound fluffy but I think it has come out at the perfect time – it’s the antidote to Brexit and its values just resonate.”

Heartfelt conversations ... The Boy, the Mole, the Fox and the Horse by Charles Mackesy. Photograph: Ebury

Mackesy confessed he was staggered by the book’s success and fears people will “suddenly realise it’s not a very good book”.

“Sometimes I wake up in the middle of the night feeling incredibly vulnerable that I’ve said these things and they’re so widely looked at but that’s fine,” he said. “I just hope it carries on helping people … Some, I’m sure, will loathe it but for people who it encourages inspires and lifts – what a privilege!”