A useful ripoff !
Subscription based applications are the best way to screw a customer, a fraudulent marketing techniques simply because, as we all know, these products reflects editions and there are no updates in the interim. So basically you pay for a subscription a price similar to the full price of the printed book and after a year they take it away. if you change device, forget about the book, if you restore the same device.. yes, forget about it. There are plenty of alternatives out there and many are not based on subscriptions or other "in-app" purchases. it is enough we are getting screwed with overpriced ebooks I don't think we should facilitate this "subscription" scam as well. Let us not forget the most important thing, all this content is found free online either though a simple search or a wiki search.