Matt Haig’s best-selling book written to guide children through troubled times has gone viral among adults too. If Christmas is a time of conflicting emotions, you’ll want this in your stocking

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For author Matt Haig, Last Boxing Day didn't start well. He was having a "bad morning" as he told The Guardian.  He'd shared his struggles with depression and anxiety with millions in his No.1 bestselling memoir  Reasons to Stay Alive  and more recently  Notes on a Nervous Planet . And so again, he put pen to paper and to cheer himself up, he wrote a story for his children. If you buy one more book this year, make it The Truth Pixie , which has sold more than 50,000 copies since publication in October and is now an audiobook  read by Olivia Colman, with profits going to Unicef. It has the simple charm of Winnie the Pooh, the lyrical genius of The Gruffalo, the wisdom of Solomon and might just lift you out of a troubled mood.

Online you can find dozens of testimonials from grown-ups who’ve bought it for a child and then bagged another copy for themselves. If you struggle with the conflicting emotions of Christmas – or of life in general, this simple fable about a pixie who always tells the truth, will not only make you smile but probably make you cry. It's readable in a single sitting and somehow, in those ten minutes, the world can seem a little more navigable than before.

The Truth Pixie is a little sprite girl who doesn’t fit in because she’s wired differently. Thanks to a curse from her Aunt Julia, she can never lie. Her high-achieving brothers and sisters all seem to be part of the shiny, happy club she can’t join, where people steer clear of the unvarnished truth because it’s not always pretty (did someone say Instagram?). And quite frankly, truth-telling is not always the most endearing party trick.

The truth pixie is a classic introvert in a world of extroverts

Eventually, she meets a little girl (skip this paragraph if you don't want to know what happens), who is beset by worries for the future: will her grandma die? Will she have to move away from all her friends? Will her family have enough money? Truth Pixie doesn’t shield her from reality, but tells her honestly that life can be hard and uncertain, but that she is a fighter, that ‘bad stuff has goods bits too,’ and ‘the bad days are the days that make you, you’.

It’s chock-full of simple truths that work as mantras for young and old. One reader posted on Haig's Instagram saying that she was buying it for her 24-year-old daughter with mental health problems, because she thinks she will love it and because Reasons to Stay Alive "was momentous for her". While the book has been described as Reasons.. for children, Haig says he wrote it not only "to help kids get through tricky moments of childhood" but for "adults who are feeling a bit down to help reconnect to their inner child".

If that all sounds a bit woo-woo, it isn’t. As my coach, the neuroscientist Dr Magdalena Bak-Maier  tells me, when we’re confused or upset and react in an extreme way or if someone pushes our buttons, we should ask ourselves, “how old is that part of you?”. Often it can be traced to a time in childhood where there’s still a vulnerable little you, whose needs weren’t met and who is crying out to be scooped up by your wise adult self. Your Truth Pixie. (Magdalena's  one-day retreats  specialise in helping you do this You're welcome).

When I asked my 11-year-old son what he thought the message of the book was, he said, “that it’s OK to be yourself”. And he’s right. But it’s also about finding your tribe, the place where being yourself is a strength. TP, as she’s called (though my son thought that was a bit too close to toilet paper) seems to be a classic introvert in a world of extroverts – parties are a disaster, she can’t do small talk (truth is ‘big talk’). However, one-on-one, she’s the wisest person you know and can be a friend for life.

I think everyone needs a Truth Pixie in their lives to reflect back that it’s all good, even when it seems bad today. Until then, we have Matt Haig. Now the only question is, how many copies of the book to buy?

Buy The Truth Pixie by Matt Haig £9.99 , is published by Canongate.