They made the #27 app by having an iPad-tuned app ready on launch day (which by definition will be the day with the fewest competing apps in any category) that

They made the #27 app by having an iPad-tuned app ready on launch day (which by definition will be the day with the fewest competing apps in any category) that probably came back at or near the top of the search results (I think we can safely assume there were a lot of searches for "free books") and whose App Store marketing text and screencaps probably looked slightly better than the other apps in its category (since on launch day there aren't enough ratings to to help distinguish the wheat from the chaff -- appearance is all a buyer has to go by). There isn't really any lesson to be drawn from this aside from the short-term benefit of being first to market in a category. And even if you find that lesson valuable, it's already too late to apply it.