In app purchase of content

Hi, We want to create a childeren's bookshop, where parents can buy more books via the web(pc/mac/nux) appstore and android market. Reading through the documentation I am wondering if this is possible with unity and how much difference is there going to need to be between android and ios code wise. The App is practically finished but we need to adapt it to take "plug-in books" somehow. At the moment it is just using a simple folder with book-content in the home folder.

The books will be a directory with the books content in them (json files, images, sound files).

Is it possible to do this on mobile, and especially where the sound is concerned because I notice the media loader (WWW) only works with OGG not MP3. I was originally planning to just buy the book and install it into the SDCard/App folder on android & PC, but i have no idea how it is supposed to work on IOS. Does Unity have some kind of bolt-on resources system?

What is the best way to go about this without having to create too much platform specific code?.