On ePub, I don't know any app that does that. iBooks lets you add notes, but not write directly into the book like on paper. It's free, download and try it and you'll see how it works. If you absolutely have to write onto a book, only way I know how is to convert the ePub to PDF and use a PDF reading app, most of which have annotation functions. If there's an app that does allow writing in an ePub book, I'd love to know about it, but I don't think it's technically possible. ePub format is just HTML with a bit of modification, there's no way to store graphical anotations data while keeping the ePub format.