This morning, Amazon released a small update to its Android app, including voice search and a Wikipedia dictionary. Hidden in the release was another interesting stat—the U.S. Kindle Store now has over 700,000 books. This is up from 670,000 books three weeks ago.

It seems like Amazon is adding books to the Kindle store at a steady pace; As of the end of April, the store had 500,000 titles. The U.K. store only has 400,000 titles, as of our last report. Barnes and Noble claims to have 1 million titles for purchase.

However, a reader pointed out that some of the books Barnes and Nobles is counting are free, out of copyright, public domain, pre-1923 books. These titles are also available for Kindle, but apparently aren’t included in the public tally of books available on the store.

Amazon claims that the latest version of the e-books are selling like hotcakes, with more next generation Kindles ordered in the first four weeks of availability than in the same timeframe for past versions of the device. And Amazon sold three times as many Kindle books in the first half of 2010 as it did in the first half of 2009.

Of course, this fast growth doesn’t mean Amazon is not paying attention to the competition. The company recently released a new ad directly targeting the iPad, illustrating that Apple’s device isn’t as easily visible in direct sunlight as the Kindle for reading.