The big question about ebooks is typically this: there's hardware, there's software, and there are titles, but how well are books selling? A good way to keep track is the IDPF's industry stats page, which was updated a few weeks ago to include January 2008 ebook sales numbers. (Note the caveats below the chart such as "Retail numbers may be as much as double the above figures due to industry wholesale discounts.")

An O'Reilly Tools of Change blog posting on Penguin Group Digital Director Genevieve Shore's presentation at the London Book Fair has additional interesting information on ebook sales. Andrew Savikas offers the following highlights from her talk:

• Though Penguin USA has been selling ebooks for 10 years, 2007 was the first time they saw "interesting revenue"

• In the first two months of 2008, Penguin USA has sold more ebooks than in all of 2007

• Readers now expect new frontlist titles to be available as ebooks at the same time they show up in bookstores

The Tools of Change page is definitely worth reading if you're interested in publishing technology. They offer feeds for the the blog, for the news, and a combined feed, which is the one I subscribe to.