In 2007 James Bowen, a recovering drug addict living in sheltered accommodation, found an injured cat curled up on a step.
"He gave me this look, almost saying, 'help', but also saying 'sort it out'," Bowen recalled.
Bowen nursed the cat he called Bob back to health, and found it following him as he busked and sold the Big Issue magazine.
The pair became known in London and attracted the attention of a literary agent.
Now, two years after publishing Bowen's story of how, with Bob's help, he got over his addictions to heroin and methadone, Hodder & Stoughton said the author had sold more than a million books in Britain.
The publisher said combined sales of A Street Cat Named Bob, its sequel, The World According to Bob, and the children's book Bob: No Ordinary Cat had reached 1,082,025 copies. "It's incredible," said Bowen, 35. "When I first saw Bob I never thought this is where I'd be today.
"This is just about me and Bob and my life, talking about how I'm not perfect. Why are people so in love with this little man who's come into my life?"
First published in March 2012, A Street Cat Named Bob has spent nearly two years in the UK top 10 bestseller list and over a year on the Sunday Times bestseller lists in paperback. It's a feat only 10 other titles have achieved. It has topped charts in the US, Brazil, Portugal, Turkey and Germany.
The story has also been optioned for a film, with a screenplay being developed by producer Adam Rolston. Bowen is working on a new series of books for children as well as a children's adaptation of The World According to Bob. Bowen's agent, Mary Pachnos, said the story "resonates around the world".
"Lots of artistic people have cats," added Bowen. "The thing about artists and cats is that cats are the owners of their own souls. They don't have a master, and artists feel that way too sometimes. A person can find a cat their muse. Bob is my muse."