Sitting with a set of felt-tip pens and a book full of black line drawings just waiting to be filled is most children's idea of heaven. But now thanks to an illustrator and designer from Tenby, West Wales, colouring in is the latest stress-busting hobby for adults. Millie Marotta, 36, is giving colouring books an adult twist with her sellout volume Animal Kingdom, filled with intricate and designs of animals filled with stylised flower shapes, patterns and shapes. Scroll down for video . Embellished elephant: Millie Marotta's colouring book features an array of intricate designs, including this elephant . Millie Marotta, 36, took eight months to complete the illustration in Animal Kingdom . The beautiful black and white book is made up of detailed line illustrations of animals, plants and birds and encourages users to 'add your own touches with colours or line drawing.' And the art book, which was published in August last year, has achieved unheard levels of success, selling 500,000 copies worldwide and been translated into 19 different languages. The success of Animal Kingdom's has come as a huge surprise, not least to Millie herself. 'My illustrations are a real labour of love, they take a long time and I think people really can and do appreciate that,' said the former art teacher. 'Because of the intricate style of my drawings people find them quite interesting and perhaps a little more captivating than your traditional colouring book. 'This is why I would say they are so well suited to adults and have proved so popular in that area of the market.' This coloured-in image of a dragonfly taken out of Animal Kingdom shows the vibrant level of detail . An image of a donkey from the colouring book is creatively constructed from flora and fauna . Millie has had a successful freelance illustration careeer with clients including Virgin Atlantic, Penguin books and Marks & Spencer, but it's her best-selling colouring book that really propelled her to fame. In 2013, she was approached by Batsford Ltd, her now-publisher, to see if she would be interested in doing the book. She took eight months to complete the illustrations in their entirety and thinks their popularity is down to nostalgia. She explained: 'I would say there is definitely a universal appeal to colouring in. 'It's something that probably all of us enjoyed as children and is a wonderfully simple creative outlet and a great way for us to de-stress in today's busy and fast paced world. A bluebird illustration has been coloured in to show the creativity and consistency needed to finish one of Millie's illustrations . This blue whale has an immense level of detail and would take a considerable amount of time to finish colouring in . Millie worked from her studio in Tenby, Wales, to create her range of detailed illustrations . 'Colouring can offer a much needed distraction from the 'grown up' world and those things which may be causing us worry or stress. A little bit of escapism if you like.' And reviewers have taken to the UK Amazon site to rave about their own experiences of the therapeutic benefits. One user commented: 'Received this book yesterday and haven't stopped since. 'I've always loved colouring but got recommended this to help overcome tension headaches caused from work. The pics are amazing an the paper quality is good. I have already completed four pics in one day!' 'Colouring can offer a much needed distraction from the 'grown up' world': The 36-year-old illustrator (pictured) took eight months to complete the book and has explained that she finds drawing therapeutic . The book, Animal Kingdom, retails at £3.99 and is currently at the top of Amazon's Top 100 Books list . The Welsh illustrator has said that, although it is mostly adults buying the book, the influence of Animal Kingdom is widespread. She has received emails from people of all backgrounds saying how much they enjoyed the simple act of colouring in her line drawings. 'From parents who have bought it for themselves to colour while their young children are doing their own colouring, to students who use it as stress relief in between study and revision,' said Millie. Many fans are  busy professionals who need a break from looking at a screen all day long but adults and children alike are buying the book as a stylish gift for friends and family. Millie says that Animal Kingdom is becoming the present of choice for people recuperating from illness, while there are, 'Ladies in their 80s who love it so much they have started their own colouring club!' Millie agrees that the rise in popularity of adult colouring books, could be down to their therapeutic quality. She explained: 'I think the level of detail in my illustrations requires people to really focus on what they are doing when colouring them. 'In that sense I think people have found them quite therapeutic and an effective way to 'switch off' as they become really absorbed in what they're doing.' The artist has also insisted that the popularity of the book could be down to the more artistic quality of the images. This image of striking stag was created by the Welsh designer out of detailed fauna line drawings . Plenty more fish in the sea! Millie's page of exotic fish has been shaded on in different colour variations . She said: 'I also think that because the illustrations do take some time to complete there is a real sense of achievement and satisfaction at the end of it. 'Someone said to me that with Animal Kingdom they feel like they are colouring little pieces of artwork rather than pages in a colouring book. 'I think people enjoy feeling that at the end of it they will have a book of their own artworks which will hopefully be something they will keep and cherish.'
Millie Marotta's book, Animal Kingdom, features detailed line illustrations . The £3.99 book is currently #1 on Amazon's Top 100 Books list . The 36-year-old illustrator resides and works in Tenby, Wales .