—Apple reported 250,000 downloads the first day, when 300,000 iPads were delivered. On April 8, the company reported 600,000 downloads and 450,000 iPads. The number of downloads more than doubled in the first week. It took the next three weeks to add 900,000 downloads, still on a triple-digit pace but the increases are pacing with device sales. We have no sense of how much the iBook is catching for repeat downloads or sales. (We also don’t know if it is adding sales or replacing them but that’s more of an e-book marketplace issue than an Apple one for now.) Then again, at least Apple is providing something beyond the number of titles available—the only stat Amazon (NSDQ: AMZN) routinely proivides about the Kindle.

—That’s an average of roughly 1.5 book downloads per device, better than the first day but not as dramatic as 1.5 million sounds overall. It’s not clear how many books that is per iBooks download; the app doesn’t come pre-installed, although Apple has said the figure doesn’t include the free Winnie-the-Pooh download offered with the app.

Milestone unit: The milestone unit was sold Friday, the day Apple released the 3G+WiFi version of its new tablet; including weekend sales, the number should be past one million. That doesn’t mean that many iPads are in circulation; the company doesn’t say whether the figure takes returns into account not does it give any number for returns.