Evidently those in the know about book publishing are aware of something called the Millionaire’s Club. To join, a book has to sell more than one million copies. Joanne Harris’s novel Chocolat, which came out in 1999 has now officially passed the magic mark. Jodi Picoult’s My Sister’s Keeper (2003) is there, too. Their books are among the 68 titles that have found their way into the Club since records began in 1998. Other British female novelists to top the one million mark are J K Rowling (Harry Potter), Helen Fielding (Bridget Jones) and Kate Mosse (Labyrinth).
It is difficult to predict what will become a bestseller. Most people like a mystery and suspense turns the pages. If a book makes readers want to know "why", most people will enjoy reading it. The Millionaire’s Club authors are commercially successful, and therefore not “literary”, but they’re good writers, producing unique, imaginative works that appeals to readers.
Carl Wilkinson’s article has lots more detail and I’m sure he’s right when he claims the median annual income of a writer in Britain today is just £4,000. Most authors have other jobs so they can continue to eat, or are lucky enough to have a husband/wife who can support them while they write. Check out the entire piece: here before you decide that writing is a good way to earn money.