Because the rest of the eBook market is so small in relation to Amazon, my advice is to use an aggregation service like Smashwords rather than publishing to each of the additional retailers individually. Not only is this cheaper because it only requires one ISBN (essentially a barcode) instead of several, but Smashwords also provides a free ISBN. Smashwords will publish and distribute your eBook through Apple, Barnes and Noble, Kobo, Txtr, Diesel, and many other smaller eBook retailers.