From the Amazon PR department:
Since the beginning of the year, for every 100 paperback books Amazon has sold, the Company has sold 115 Kindle books. Additionally, during this same time period the Company has sold three times as many Kindle books as hardcover books. This is across Amazon.com’s entire U.S. book business and includes sales of books where there is no Kindle edition.
As a like-for-like comparison, the first sentence — the one highlighted by most reporting — clouds more than it clarifies. A bit of basic algebra (100p = 115k, where 3k = h) fills in the blank, giving a paperback-to-hardback ratio of 60:23, but the phrasing makes me wonder whether Amazon is trying to be coy about which market the Kindle is aimed at, and which market it’s eating into the most.
Unlike much of the anglosphere, the North American market remains tied to the higher margins and royalties of first-release hardback editions, even though many of the works that appear in that format are semi-disposable, and would likely be bought as eagerly in paperback with less weight and less pomp. Instead, the reader has generally been posed with a choice of paying the hardback premium and enjoying the collective experience of a new work, in cinematic fashion, or waiting for a library edition or paperback, by which time the social moment may have passed — or, in the case of nonfiction, the content may already be dated. 
Heavy discounting from chain bookshops and online retailers has blurred the pricing gap between the formats: since the list price is best regarded as a convenient fiction, sucker’s premium or anchor round the neck of the independent bookshop, you can readily buy new hardbacks for the cost of a standard trade paperback. What hasn’t blurred is the physical gap: while booksellers might like to perpetuate the idea that buyers are getting hardback quality for bargain prices, I suspect the more likely perception these days is one of ‘paperbacks with excess baggage’, with American readers increasingly feeling like Allen Lane at Exeter station.
Enter the Kindle. Or, more precisely, enter the e-reader at a tempting price, with a compelling bookishness, and a publishing format that disrupts the conventions of the American market with subtle devastation.
Take, for instance, the latest Stieg Larsson: with no US paperback currently available other than the large print edition, the choices on offer from Amazon are:
- the ‘deckle-edge’ hardback at $14.26 (around half the $27.95 list price); 
- import paperbacks from third-party sellers at around $18; 
- the Kindle edition at $9.95
(Barnes & Noble doesn’t delve into the grey market, but it matches Amazon for the hardback, and offers a Nook version for $9.99.)
If the consistency of your shelving depends upon Hornets’ Nest lining up pleasantly with the preceding volumes, deckle-edge and all, then the hardback is the obvious purchase. But for regular readers of semi-disposable books, the electronic edition delivers paperback pricing on a hardback release cycle without the hardback’s bulk — enough that the device conceivably pays for itself in short order. 
Which brings me back to that press release. Given that most new hardbacks sold in the US market are little more than steroidal paperbacks, comparing the sales of Kindle editions to the overall figures for paperbacks or hardbacks is statistical chaff. There’s one ratio that really matters: the number of $10 Kindle editions sold for every new hardback that’s listed around $30 and sold at around $15.
So, why that phrasing, emphasising the Kindle as an alternative to paperbacks? Is Amazon deliberately burying the lede to remain in the good graces of the publishers on whom it depends to supply timely electronic editions, even as e-readers seem poised to decimate the market for new bestselling hardbacks?
I honestly don’t know, but it comes across as the most delicate of dances.
1. In further emulation of the cinematic model, publishers now load up paperback nonfiction with revisions and extras in the hope of generating a second sale.
2. ‘…for books with deckle-edged pages will doubtless continue to be produced as long as the book-buying public lacks knowledge and taste and as long as there are publishers who are willing to take advantage of that lack by supplying a demand which most — not all — of them know is misguided and reckless.’ (New York Times, March 23rd 1903)
3.> Like the Harry Potter series, Larsson’s books have been shipped in from the UK; unlike the later volumes of the Harry Potter series, the aim is to get ahead of the American release schedule. Amazon ‘respects’ the publishing fiefdoms of the English-speaking world (and 17 USC 601) by not selling import copies itself before US release dates, but doesn’t impede private purchases: another delicate dance that electronic editions will complicate further.
4. Yes, I know, you can’t pass e-books on once you’ve read them without breaking the DRM, but are hardbacks really loaned or given away as often as paperbacks to begin with? You might sell them for a fraction of the cover price, or donate them and take a writeoff, but the formality of the form conveys an implicit rights management — another reason, I suspect, why American publishers cling desperately to hardbacks.