For more than a decade, Matthew Johnstone hid his depression like a dark secret. But after spending most of his adult life struggling to accept the condition, the 44-year-old author and illustrator learned to embrace "the black dog".

The metaphor for depression, made famous by Sir Winston Churchill, is depicted in a picture book, Living with a Black Dog, that Johnstone has produced with his wife, Ainsley. Published this week, it features a forlorn-looking black labrador overshadowing people with depression - giving a physical form to a condition that Johnstone describes as "a loss of joy".

One of the aims of the book is to emphasise the importance of talking to others who might be able to offer support. "People are too afraid to say how they feel or are worried what people will think," Johnstone says. "It's important that people talk about their feelings in facing up to their problems. It's important to realise that they are not alone."

Alongside his cartoon-like illustrations, the book offers practical and humorous tips on helping those affected by the condition. One arresting image of the black dog pulling a man's socks down is accompanied by the caption: "Socks have little to do with mental health."

Johnstone says his book offers a different approach to traditional self-help texts. "People who are depressed find it hard to concentrate, so a picture book makes this complex subject much more accessible," he explains.