The additional content varies with the book. For a thriller, the reader might get an animated graphic novel, while a kid's book might have a game readers can play. Which brings up a point. The publisher's FAQ points out that “The extra content in Book and Beyond ebooks, at present, will only work on PCs or Macs using Adobe® Digital Editions.” Ergo, this is one evolutionary step that might well fizzle. PCs have been available for ages and only small numbers of people were willing to read books on them. Apparently, the files can be downloaded to eReaders like Sony (no mention of Kindle, as this is a UK-based effort; an iPhone version is planned) but because they are so large, the user can't fit nearly as many enhanced ebooks on the eReader.
Time will tell whether this catches on. It would seem to me a better solution— until we have an eReader that can play the added content and still keep the more readable e-ink screen— would be to have the additional content web-based and merely provide the ebook buyer with a user name and password to get to it. Otherwise, eReader owners will have to give up a lot of space for something they can't use.