The ebook editions are available in ePub format to load on a wide range of compatible devices, and so far Pottermore has partnered with Sony Reader (its primary partner; only for US and Canadian customers), Amazon (stores worldwide, in English), Nook (US and Canada), Google Play (US, UK, and Australia) so that the ebooks can be sent to a customer's linked account at any of those services. Those retailers will receive an unspecified portion of sales to their linked customers. Pottermore is providing the files without DRM, but with a personalized watermark.
The ebooks are not currently available for direct transfer to iBooks (or other services such as Kobo) because Pottermore and Apple didn't reach an agreement prior to the ebookstore launch. Pottermore ceo Charlie Redmayne tells the Bookseller "we hope to be working with all partners going forwards, including introducing an affiliates scheme." The ebooks are available only in the US and UK for now, but French, Italian, German and Spanish editions will follow in the next few weeks (and more languages to come later.)