A. Nook and Kindle are warehouse application sets that are intended to control access only to approved merchandise, and IStore is doing same thing.
B. Epub is industry standard, and one that Android(which is racing past the other three in e reader and smartphone usage) uses Epub approach.
C. Adobe Digital Editions and Reader are compatible with EPub and 95% of installed desktops and laptops are using one or both of these products(that number of potential readers is NOT less than 150 Million potential users).
So moral of the short story is this. IGNORE installed base of potential readers at the peril of Your ebook sales success, AND use the Niche approach to reach a focused market of HIGH potential pass along readers.
BTW, if You have a PC and not an ereader tablet or e reader, we have a guide for downloading Free E readers for Your laptop or desktop computers. http://bit.ly/100MMPCEbook
Hope that perhaps this FreEbook about Socially Responsible Ebook Marketing can be a good resource guide. http://bit.ly/EbookInYourVoice
And then we have the Ebook4Good edition which comes with Hangout support here http://bit.ly/SocRespEbook2013
Or As a Gift For Your Socially Aware Community Writer, Author, Self Publisher or Activist http://bit.ly/EbookNicheGiftSW2013
Please consider joining our Google + community for weekly Hangouts4Good http://bit.ly/Ebooks4Good