How much is the iPad cutting into Amazon’s ebook business?

Exactly how much is the iPad cutting into Amazon’s ebook business today?
That’s the magic question, which Laura Owen does a good job of teeing up over at paidContent. With the release of Amazon’s most recent quarterly numbers, it’s clear that the company is still in a rising-tide market, where its boat — along with a few others — is still heading upward.
Truth be told, exactly how much Amazon will be impacted by rising tablet readership numbers is practically impossible to determine at this point, in part because the growth of the overall market is blurring any potential abandonment of Kindle as a platform (and by Kindle, I mean hardware, software, and storefront). And while we know there is some abandonment, the company has done a good job of locking in consumers to its storefront, including many of those who abandon Kindle hardware (which means many current iPad users are actually still using Amazon’s Kindle app to read ebooks).
So while it will be hard to determine how much the company could potentially lose as tablet readership rises, it’s still worth examining recent survey data showing that the Kindle has a lower share of the primary e-reader in multi e-reader households.
Before we look at those numbers, let’s first look at the total adoption of e-reader by brand, according to a recent survey by my firm, NextMarket Insights.
 Figure 1. E-reader penetration by brand

E-reader penetration by brand

Source: NextMarket Insights

Looking at the data, Kindle e-readers (nontablets) are in about half of U.S. e-reading households, while Kindle Fires are in nearly a quarter . Amazon should be commended for a good job here in answering the tablet call itself.
Apple, on the other hand, is in about one-third of e-reading households, about what is probably expected at this point in the market.
So with that in mind, let’s look at share in multi versus single e-reader households.
Figure 2. Primary e-reader (single vs. multi e-reader households)
Primary e-reader (single vs. multi e-reader households)

Source: NextMarket Insights

According to the survey data above, a quarter of e-reading consumers in multi e-reader households (which is about half of all e-readers households today) list the iPad as their primary e-reader, a share over twice that of single e-reader households.
What does that mean? Well, single e-reader households are likely still on their first device, and that translates into much lower adoption of the iPad. In multi e-reader households — households where many consumers have moved onto a second e-reader or bought a tablet — the iPad is performing much better in terms of primary e-reader share.
This all illustrates how the iPad’s growth is definitely a threat to Amazon and Kindle. But exactly how much is this impacting e-book sales through Amazon’s storefronts?
Like I said, that’s still impossible to tell.