Yesterday, The New York Times published an article describing the hot sales of e-book romance titles. The NYT attributes high romance sales partly due to e-readers (like the Kindle) functioning as a sort of "brown-paper bag." This allows the romance-lover to read discreetly, without strangers on the bus commenting. The paper bag theory might also explain the popularity of the racier strains of erotica.
I think the NYT's conclusion is pretty interesting, considering that romance book sales are driven (or thwarted) by the quality and excitement of the cover.
Evidently in the e-book market, the cover might seduce, but once a title is purchased, the cover is kept hidden.