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To run calibre you need a device with windows, os or linux and each version is different. Not that helpfull for different devices.
Most devices recognize which app can open a file. I would think, that is much more easy than a complete software. Oh, and I would assume, that someone who buys an usb-stick knows how to use it.
DRM you won't get outside of big vendors or high cost. If you are concerned by that, only sell through amazon. But I don't think any pirate would buy an usb-stick with the file, if he can much more easily crack the amazon drm.
I don't know amazons contracts, but I would check if you are allowed to sell your files yourself, if you are in an exclusive contract.
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