Boy wizards, sparkly vampires, BDSM - and now colouring books. Publishing trends are nothing if not unpredictable but is there any further substance to the most recent one beyond trying to colour inside the lines?
It seems as though everywhere you look, someone's clutching an adult colouring book - except at Whitcoulls, where the most popular titles, by Johanna Basford, are completely sold out and on back order. Booksellers are struggling to keep up with demand as New Zealanders clamour for colouring books for grown-ups.
For the millions worldwide who are rushing to their pencil cases, there are as many reasons to colour in as there are ways to fill in the picture: switching off; being more present; or simply keeping your hands busy.
Six of the top 25 books sold on popular bookselling site Book Depository are adult colouring books, while the most popular ten colouring titles have sold almost 25,000 copies in New Zealand, according to Neilsen BookScan.
It's not a slow-burning trend so much as a big jump over a short space of time.
"Everyone's noticed the spike. It's a recent gone-crazy thing - and who knows why, it's like any trend in publishing," said Neilsen Sales & Marketing Manager Nevena Nikolic. She said the colouring books were pushing other books out of the non-fiction bestseller lists.
It began with Ms Basford, a Scottish illustrator who had previously worked in advertising and textile design. Asked by a British publisher to draw a children's colouring book, she pitched the idea of one for adults - Secret Garden - which went on to sell millions of copies worldwide.
"I've heard from so many people ranging from lawyers, financial advisers, business owners and busy mums, [and they] all say the same thing: that colouring in helps them relax.
"A blank sheet of paper or an empty canvas can be daunting, but a colouring book acts as a bit of a buffer in this situation," she told The Guardian.
New Zealand intermediate teacher Lowri Ifor uses a children's colouring book, and agrees that it's a good way to de-stress.
"I find it relaxing - it's just an easy way to switch off. I've only started recently, after I saw so many on sale in the shops, but I'm really enjoying it.
"Given how much I find myself doing short tasks all the time, I feel like it's good to concentrate on something for a longer period, and there's a sense of achievement when you finish one. We've been putting them up, but only as a joke to annoy my housemate who doesn't like them - I don't actually value them as 'artwork'," she says.
Booksellers are riding the wave of what chief executive Lincoln Gould is calling a "craze."
"They are an absolute phenomenon," he said.
But why are they so successful? Some publishers and booksellers are promoting them as mental health support, with Amazon listing the books under "self-help".
The Zen Coloring Book series - Color Me Happy, and Color Me Calm - are authored by an art therapist, and promise an improvement in mood. And in the US, the PTSD Survivors of America have embraced the trend, in August encouraging people to "Color Across America for PTSD Awareness" for National Colouring Book Day.
The bestselling Mindfulness Colouring Book promises to soothe anxiety and minimise stress through its pages. But Mental Health chief executive Judi Clements was quick to make the distinction between colouring in and practising the popular technique of mindfulness.
"The book is just a book, but it might help bring about a state of mindfulness. And it has been shown that colouring in can help decrease anxiety and depression. Basically, if you're present when you're colouring, you have a sense of curiosity and openness, and you know what you're doing, then that quality of attention is mindfulness," she said.
"Mindfulness is not just about being focused - after all, snipers are focused. It's about having that sense of kindness, openness and curiosity, which colouring books can help with."
Auckland-based art therapist Josie Scott spoke positively about the books, but said that though some people did find them relaxing, they couldn't be called art therapy.
"Art therapy is about a three-way relationship - you have your client, the piece of art, and the therapist. It's a completely different process, but what it does do for many adults is that the thought of Art, with a capital A, can be very scary. If it takes away some of that for people, and gives them an opportunity to be playful, then I think that's great," she said.
The trend has led two Kiwi friends to set about producing a book of their own, featuring New Zealand flora and fauna.
Inspired by the success of the books overseas, illustrator Cat Fawcett Cornes and entrepeneur Maree Glading hope to release the book by the end of the year, which is being funded by a PledgeMe campaign.
"I'm a busy mum: I have my own business, I have two small children, and I had been exploring mindfulness and the benefits of living more mindfully," said Ms Glading.
"Colouring books really help with anxiety and being more present, and we saw a gap in the market. No one at the time was doing anything New Zealand-based, which seemed a good idea because there are so many beautiful New Zealand images.
"I often colour in with my kids - they have their books, and then I'll colour in with them in mine - it's just a nice thing to do, and if you do something really beautiful, you can frame it or put it up.
"Not everyone can draw, and it's really nice if you've got the picture there and you can put your stamp on it," she said.
An aid to mindfulness, a comparatively cheap option for wall decor - or simply a sign that too many of us need a proper hobby? Whatever the answer, for as long as the craze lasts, bookshops are cashing in.