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Lexcycle Stanza is the most mature and sophisticated eBook reader application for iOS. Originally developed for iPhone, the product was purchased by Amazon in 2009.
Stanza allows you to purchase EPUB and MOBI format books from several sources and also allows you to import your own EPUB and other file format content thru iTunes or via the Open Source Calibre Content Server application.
Here is the content description for a book in Stanza.