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A computer and any particular portable reading device may have various reader applications available for that computer or device. Various reader applications can handle various eBook formats. Various vendors sell various eBook formats, reader apps, portable reading devices, computers.

I use calibre to manage all my eBooks regardless of the vendor/distributor source. Calibre remembers which format I like to read on which device when I send books from calibre to that device. While sending, calibre automatically converts books to the format for that device if that particular format doesn't exist in my calibre library. I use calibre on my computer to store all my eBooks with one or two master copy formats per book. Using calibre I delete all other copies of eBooks from my reading devices after reading them on whichever reading device.

I usually use the calibre viewer (reader app) when assessing the book after it first arrives on my computer, prior to clean-up if necessary, then send books I want to read to the appropriate portable reading device to actually read them on that device.