The erotic novel Fifty Shades of Grey has become the fastest selling paperback since records began and is on course to sell one million copies by the end of 2012. Beating JK Rowling's, Harry Potter series and Dan Brown's Da Vinci Code to the UK record.

The book written by first-time novelist Erika Mitchell, using the pen name EL James, has been dubbed 'mummy porn'. The book with its graphic sex scenes has become the quickest selling paperback to reach £ 550,000 sales. And it's only the fourth paperback novel to sell 100,000 copies in one week. It is also on course to spend its fourth consecutive week at the top of the official UK top 50 book chart.

Its explicit sexual content led to it being banned by some libraries in the United States, which further fuelled publicity for the novel following online petitions and an outcry from free speech groups.

So it's true sex does sell. And it seems the saucy book with its distinctive cover, is a dead giveaway, that let's others know what you are reading on the way to work. So fans travelling around have taken to wrapping brown paper over the book hiding the cover. So when you next see someone with that that distinctive paper, give them a knowing smile that says, I know what you're reading!

Written and Presented by Ann Salter