That's 'E' for Erotic and 'E' for E-books.
Publishers Weekly article on "The New E- in Erotica"
The popularity of erotic romance/erotica is hardly "new", but the article is definitely worth reading. We're quite happy that Ellora's Cave gets several mentions, including: "The be-all and end-all of online erotica, says Peters, is Ellora'sCave."
And an article in the Washington Post on projected Amazon sales of e-books:
"Amazon E-Book Sales to Hit $2.5 Billion in 2012"