'The success is still sinking in'

Biscuit crumbs are gently brushed away, and she's resolved to eat lunch at the kitchen table on a more regular basis.

This is because her desk is frequently covered in her painstakingly detailed illustrations, which have been packaged into her massively popular adult colouring books to meet demand for a growing trend - one that looks set to continue this year.

"If I'm drinking tea or coffee at my desk, I make sure it's far enough away that if I knock it, it's not going to go all over the drawings," she admits. "I'm quite careful!"

Each page takes her at least a day to create. She uses a Rotring Rapidograph steel-nibbed pen and marker paper.

What if she makes a mistake?

"I wince," she says with a laugh. "If it's just a wonky line or slightly imperfect shape, it doesn't matter too much because it's a hand-drawing. But if I get half way through and mess up, I have to start again."

Her first book, Millie Marotta's Animal Kingdom, was Amazon's bestselling title of 2015, outstripping E. L. James and Harper Lee, selling almost 500,000 copies by Christmas Day.

"It's still sinking in," she says of her colouring book success. "It's been such an incredible journey but at the same time it's bizarre, because for me, day-to-day, I'm still doing what I was doing before.

"I work from the same room at home and I have to pinch myself sometimes, to remind me that actually, things have changed quite dramatically.

"I was surprised at the phenomenal success," she continues. "All I had in my mind with the first book was that I hope people enjoy it."

Animal Kingdom was followed by Tropical Wonderland, and now Wild Savannah makes it three, inspired by her love of the natural world, featuring swaying grasslands, birds and beasts - all waiting to be coloured in and embellished, to ease away the stresses of life.

Despite the positive reaction to these gorgeous books, Marotta, 36, still seems a bit like a rabbit caught in the headlights, and can't quite believe that all eyes are on her in the world of publishing, as she endeavours to continue her quiet life in her three-bedroom cottage in Tenby, south-west Wales, where she creates her bestselling illustrations.

She lives with her partner, Hiut Denim wholesale and repairs manager Paul Anderson, in the same rented home they've shared for years.

"The spare room has become my studio. The second bedroom is where all the stuff gets dumped. The bank balance is much healthier now, which takes a little bit of getting used to, but it's amazing. I never anticipated the books doing as well as they've done."

She hopes they will enable her to buy her own house for the first time in her life, with more land and enough room to have pets. That's when the colouring success will hit home, she anticipates, although she says she's not a millionaire yet.

She may shy away from the spotlight, but her boyfriend is relishing Marotta's new-found fame.

"He's a big show-off, he loves it! I've never been particularly comfortable with being the centre of attention, so this has taken quite a lot of adjusting for me, but he thinks it's fabulous."

Since her colouring books took off, she has received thousands of letters, emails and Facebook messages from people praising her illustrations, and thanking her for helping them relieve stress, or get through difficult times.

"Some of them are incredibly personal and catch me off-guard," she reveals. "Lots of people suffering from anxiety or depression find colouring to be a good distraction when they are feeling really low.

"Colouring is a way to slow down and switch off. Today, we lead such busy, fast-paced lives with multi-tasking and trying to cram so much in. The colouring books have just been an enjoyable and accessible way to encourage people to stop, slow down and switch off all the distractions in life."

Born in Bedfordshire, the daughter of Italian hairdressers Antonio and Lynne, her parents divorced when she was five. Along with her three siblings, Marotta's mother took them to live in a cottage in rural Wales, where they'd previously spent family holidays.

"My earliest memories are of growing up in the countryside on a smallholding. It was a very outdoorsy upbringing and I was a child who was nutty about nature. We used to go to Italy with my dad to visit his side of the family in school holidays. I remember spending our days on the beach and climbing hills, which we all really enjoyed."

Her mother would draw and paint as a hobby, which sparked young Marotta's interest in art. "I was a very creative child, always painting and drawing," she recalls.

She eventually studied wildlife illustration, before teaching art at a local comprehensive school for five years.

"I really enjoyed teaching, but as I was nurturing other people's creativity, I found more and more that I was missing doing that for myself. It got to a point where that balance tipped, and I realised I wanted to spend all my time doing my own artwork."

So she quit her job.

"Leaving a secure, relatively well-paid job was quite terrifying," she admits. "It wasn't a decision I took lightly and I saved money for a couple of years, in case I needed something to fall back on."

She didn't need a fallback plan, though, quickly landing work as a freelance illustrator, for brands including Virgin Atlantic, Penguin Books and Marks & Spencer.

In 2013, Batsford got in touch to see if she'd be interested in doing a colouring book.

"Animal Kingdom took me around eight months, although I was also working on other projects at the time."

Today, she doesn't have time for any other projects. Wild Savannah is about to be published, and she's already working on her fourth book.

"It doesn't feel like work for me," Marotta confesses.

"When I buy my first property and have a bit more space around me, and maybe have a studio that isn't part of the house, that will be when the success of the books will hit me."

:: Millie Marotta's Wild Savannah is published by Batsford on February 11, priced £9.99