Apple uses ePub for iBooks, but they have Apple's Fairplay DRM (encryption) which has not been circumvented. So they are not transferable to non-Apple devices.
Since iBooks does support DRM-free ePubs, your best bet is to buy from B&N or anyone selling Adobe ePubs. Then strip the DRM for use via iBooks (or use a 3rd-party ePub app).
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