It is a sales aid, based on tweets I have made for the time poor sales professional.
Still appreciate other places though please!
How about Twitter?
Seriously though, I'm assuming that you have productised your content into a commercial (not free) app, and you want free marketing methods.
I would actually turn that question on its head and say that apps are sometimes better used as the free marketing method for some other commercial product. In this case the app should be free, but somehow entice the user to go for some other paid for product.
One example is having a cut down free version of your app, which may entice people to go for the full paid for version of your app.
Another example is having a cut down free version of your app, which actually advertises a totally different product, for instance your full content productised in a book format, physical book or ebook, or your content productised in a website subscription service model.
I hope this helps, and I'm actually in the process of following some of my own advice with my company's latest game app too.
Other free marketing options include:
1. creating a website, or section of your existing web, to market the app
2. videoing your app and putting it on youtube, e.g. like this
video we created for our app
3. whichever channel you use, including the app store itself, make sure you use the right words in title, descriptions etc, so that people searching for your app can find it