Goodereader.com has a post about JK Rowling bringing two of her suplemental books about Harry Potter to ebook format for charity. Two books (Quidditch Through the Ages and Fantastic Beasts and Where to Find Them) were originally written as fundraiser books for the charity Comic Relief. They are now are being brought to ebook and their proceeds will again to go the work of Comic Relief.
It is worth noting, as goodereader.com does, that JK Rowling is the first billionaire that has lost her billionaire status as a result of giving away so much of her wealth to charity.
The two books, along with all of the rest of the Harry Potter books are available exclusively on ebook at Pottermore.com