As of today, Harry Potter ebooks are available for consumer purchase exclusively via the Pottermore.com shop.
As of tomorrow at 9 a.m., ET, the entire series will be available for checkout from OverDrive, according to an email sent by the company earlier today. Pre-orders, which had been temporarily halted, have now been restored (and the 10% discount extended through 4/30). The company also says that all holds place on earlier ordering will be honored. The HP books are available in EPUB, Kindle and MP3 audio formats.