My theory with content apps (I've seen this elsewhere) is that the majority of money comes from a small percentage of users (1-10%).

These users are what typically is referred to as whales. The biggest obstacle to monetizing whales is new content. Also, whales tend to be less price-sensitive. That is, they will buy what content you have at a reasonable price, and that price should be based on what those (paying) users will pay.

I don't really buy the convenience argument. It's unclear whether they released 10 books and users were still only buying 2. That is, they were unable to test whether convenience or the lack of new content was the gating factor. I suspect it was the latter.