You can easily list all books that has a epub format, and delete all other formats. Or all Mobi books and delete all other formats. And so on. So it is pretty quick, even if you have to go through all the formats in order of preference.
I list all books that lack a epub format and try to create one. And I keep all other formats. You never know....
I only read using epub format or PDF, if I have to.
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