Are sales of books plateauing? Slipping? Increasing?

Russell Smith reports that, in recent months, sales of e-books are plateauing and even slipping, while sales of paper books are advancing after years of decline. Setting aside the fact that industry figures only include book sales from established publishing houses and not the burgeoning trade in self-published books (which are mainly in electronic format), this still seems like it might be something of a watershed in the now age-old argument of e-books vs. paper books.
The reason for the slump in e-books is not readily apparent. It may be because e-book prices have been maintained at an artificially high level (which may, or may not, now change). Or, more worrying, it may be a sign that the whole industry is in a downturn.
Most readers have an opinion on the electronic/paper book debate - personally, I prefer old-style paper books - but I'm sure that no one want to see reading as a whole decline.