It turned out to be a highly prescient decision to feature EL James' Fifty Shades of Grey as our 'killer app' last week. This week the book has been making yet more headlines as it became the fastest adult paperback to sell one million copies, breaking the weekly paperback sales record in the process. Sales initially grew via word-of-mouth, and subsequently the erotic subject matter and descriptions of the book as 'mommy porn' garnered this publishing phenomenon an enormous amount of coverage. EL James even appeared on Newsnight a few weeks back. Now sales have gone through the roof. What's super-interesting to me about Fifty Shades of Grey is that when I first started hearing about the title's success, it seemed to be all about the ebook version. I assumed quite reasonably that this format was the perfect option for those readers who didn't want to publicly proclaim their choice of steamy reading matter. But tie me up and cover me in whipped cream, look what's happened – who could have imagined that physical book sales of the title would take off in such a dramatic fashion? The presses at printer CPI are not so much grey as red hot. EL James and Random House have achieved something quite remarkable in taking ladies' naughty reading mainstream. Last night I saw a woman clutching a copy on her way out of the tube station. No need for the proverbial plain brown paper bag. Just last week former labour communications honcho Alastair Campbell tweeted, thus: "On train north. 3 women reading 50 Shades of Grey. Bet they wouldn't like it if I whipped out porn mag. Me I'm reading @thejeremyvine v good" My response to @campbellclaret was that this is a book, which makes it a rather different proposition to a copy of, say, Razzle. And a host of other publishers have jumped on the bandwagon with titles targeted at this previously untapped appetite. What a great affirmation that actual books can still be sexy.