Harry Potter is one of the most successful publishing phenomena of our time, selling 450 million copies. Its success has transformed author JK Rowling from an impoverished single mother into one of Britain's richest women. Since The Deathly Hallows was published in 2007, Rowling's fans have been desperate to know what she was going to do next. The answer is The Casual Vacancy, a novel for adults with some very grown-up themes. The expectation and pressure are enormous.