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Full user review
with each edition this becomes more useful.
Installs on Linux, too!
Maintain a library accessible by all your devices.
converts to epub for most devices, even from plain text, and you can add your own cover!
Occasionally an update will break a feature for a specific device. Turn auto updates off, and read about each update to see if you would like it, since bug fixes might be issued as well
This is more useful than the "kobo desktop app" or any of those, and does NOT LIMIT you to shopping at Amazon or a single store, it also allows you to simply download free e-books from any site on the web and then add them to your collection without any tracking by Amazon, Google, Kobo, etc.