Vook are in the process of proving one theory I had about enhanced ebooks, ebook apps, and so on. While I am not sure there is a profitable market for enhanced ebooks, I do believe that there is a market for a paid service to make enhanced ebooks. The difference between offering a service and releasing their own titles is that Vook won't be risking as much of their own capital in making an enhanced ebook. They will get paid based on someone else's risk.
There's a market for the service because only certain titles are going to do well and it takes a content expert (author, publisher, or marketer) to identify what will add the best value to an enhanced ebook. So if Vook plan to let an outsider direct their actions then it would be best to treat this as a service.
That brings up a second point. Even though I don't think there is a general market for enhanced ebooks, I still think there might possibly be one for specific titles. That market would exist not as part of the enhanced ebook market but as an extension of existing fan base. For example, Pottermore could release an enhanced HP ebook with all sorts of extra content and I'd think the book would have a good chance of turning a profit.
Vook had actually been making moves toward this goal for most of a year now. They announced MotherVook, their ebook creation tool, some time ago. This is a tool that any of Vook's parnters could use to create their own enhanced ebook. The only change to day is that now Vook will focus all their attention on the partners and none on creating content internally.
All in all, this is an interesting shift for a 2 year old company. They started out making apps just for iOS, and now can help make enhanced ebooks for Kindle, iOS, Android and I've gotten hints that they are working on distributing onto Chrome.