You can read all about it here. It's part of her new website, called Pottermore, and the new e-book distribution shows that she either re-acquired the electronic rights to her books from Scholastic or was savvy enough not to sell them in the first place.
Her books were probably among the most electronically pirated books on Earth and she's lost God knows how many millions of dollars as a consequence. Now, I hope, she'll be appropriately compensated for her work, irrespective of how it's acquired. I'm also sure many fans feel that it's about time Harry and company were legally available for download.
E-books are a fast-growing market and the market is likely to continue growing for years to come. Whether the market is large enough to professionally sustain dozens of self-published novelists is yet to be determined but I'm optimistic that this is the case. If nothing else, this news will doubtless spur a spike in e-reader sales. This can mean only good news for those of us published on the likes of Kindle, Kobo, Nook, and elsewhere.
The real news for fans, though, is the new website itself, which promises not only news about the characters but offers, in a partnership with Sony, a new interactive game that's likely to keep fans returning for years.
All in all, a very savvy move for the world's richest novelist. Harry's been very good to her. I hope my characters are even 1/100th as good to me.