The ebook catalog in Amazon’s Kindle Store for U.S. customers has just passed half a million titles. Here’s a link to the Kindle Store’s book listings, which numbered 500,461 as of 10 p.m. EDT Sunday, April 18, 2010. There were fewer than 90,000 Kindle books when Amazon launched the Kindle on November 19, 2007.

Adjust free public domain titles out of the titles listed for both venues, and this leaves the Kindle Store with, roughly, a 16-to-1 advantage over the commercial title listings of what is now seen as its leading ebook retailing rival, Apple’s iBooks Store. (This extrapolation is based on the fact that Apple launched its iBooks Store with 60,000 titles, half of them free public domain titles; the figure may have grown since but the iBooks Store is not available for the kinds of search, browse, and sort functions that would make it easy to check).

Although some might use that temporary metrics gap as a basis for concluding that this much-hyped rivalry is rather like the rivalry between the hammer and the nail, that’s not how I see it.

The iPad is a lovely and — from the point of view of a competitor — formidable ebook reading environment, and here at Kindle Nation Daily and our sister site iPad Nation Daily we’ll continue to cover and illuminate its reading-friendly features with various applications including the free Kindle apps, iBooks, and other apps. Continued competition is bound to improve both companies’ devices and their reading and listening apps, and we’ll be paying close attention and making plenty of suggestions along the way. From where I sit, each company needs the other, but more on that later.

Meanwhile, it will be interesting to see if Amazon issues a news release on this major benchmark, or lets the news pass quietly in the night.