There's a little bit of a rush to bring content into app form. It's so easy to make an ebook and cram it with videos, or to turn your how-to guide into something that looks slick on an iPad. I think of these electronic projects as the new coffee table books. Beautiful, but unfortunately, not widely read.
The problem is this: when you turn this work into an app or augmented ebook, you're not moving to a less crowded market, not moving to a place where you will earn more attention from strangers. In fact, unless your app is connected, portable and/or aware, the only people who are likely to use it are people who were already your fans. (Not that there's anything wrong with that, if that's your goal.)
Connected: the app works better when other people are also using it. Like the fax machine (what did the first owner of a fax do with it?), these apps have a hurdle at first, but get more and more appealing as the word spreads. Instagram and Twitter are connected.
Portable: sure, PDFs and paper books are portable, but there are certain forms of content where having the content in your pocket is really useful. I'd put frequently updated, timely content (like the weather) or content I'll need to refer to again and again on this list.
Aware: Our mobile devices know where we are, and in some cases, know what we've just done. Telematics opens the door to a huge number of breakthroughs, only a few of which we've seen exploited to date.
Slick is not the goal. I know that apps are shiny and new and sexy, but if your goal is impact, you'll need at least one of these three elements--or you're better off in a different format. More than a decade later, email and free-to-share digital text and video remain killer apps if you're trying to spread the word.