In what is seen as the ultimate evidence of adults seeking "digital detoxes", the 'Secret Garden' has sold 1.4 million copies in 22 languages since its release in 2013.
The adult colouring book contains intricate black and white floral patterns, birds and other images of nature.
Basford's latest colouring book, 'Enchanted Forest' that was released in February has also sold a record 226,000 copies already.
"Brilliant. Just great for loosing yourself," wrote an Amazon reviewer.
Another reviewer wrote: "The family has been under a lot of stress lately ... Realising now that this love of colouring in has never gone away has given me a lot of comfort. It makes such a change from browsing the internet or staring at some rubbish on TV that never actually alleviates stress. Colouring-in does, believe me."
Basford, originally a silk-screen designer, was approached by Laurence King Publishing in 2011 after the company saw some of her work online.
Despite the company wanting Basford to do a children's colouring book, she insisted on an adult colouring book –a concept very rare at the time.
Since then, the trend has picked up with a rise in the sale of adult colouring books as more and more people look towards colouring as a stress buster.
Forty-four-year-old accountant, Louise Woollam, started colouring after losing her sense of smell.
"I had an enjoyable hobby as a fragrance writer previously," Woollam told IBTimes.
"I've been practicing mindfulness as a way of dealing with the depression that arose from my condition, and these seemed like a nice, creative accompaniment to my other therapies."
Woollam uses a series of books titled 'Art Therapy' that feature the drawings of Richard Merritt.
"Unlike other therapies, there is something tangible, and visible, at the end. I like the complexity of the patterns and the puzzle-solving nature of working out color combinations," said Woollam.