There's a long tradition of such books that captivate, titilate and then vanish without a trace. Take Valley of the Dolls as an example, or anything by the likes of Jackie Collins. I doubt whether these books pose a serious threat to novels.
And they are a long way off being the best selling books . Harry Potter sold at least 450 million while these have somehow stretched to 40 million. A long way to go there and yet people keep pointing out that its outsold the series . Thats less than 10% of the way there and the HP figure doesn't include Ebooks
'There is no future for e-books, because they are not books. E-books smell like burned fuel.' Ray Bradbury (RIP)
It's just a modern "romantic" novel with bondage. I read a sample to see what the fuss was about, honestly i still dont get it, the writing was so dull. However, I don't think other writers need worry too much. Books have never before been so accessible.