Well I downloaded the 15 that were free. Probably they track which feature you use the most. Like seeing if you use the hotel contact info. to contact hotels, for instance.

If it gives them ideas to make more interactive features -- like maybe they could collect a commission for hotel reservations that these apps. generate -- maybe they can offer this content for free all the time.

One nice thing about this electronic content, besides being so portable, is that they could give it away for free as a promo for a temporary period. Can't do that with physical books.