Based on our record, Alfa eBooks Manager seems to be more popular. It has been mentiond 2 times since March 2021. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
Alfa eBooks Manager (site seems to be down at the moment; more bloated than Calibre, but at least it let you keep your folder structure). Source: over 2 years ago
Not FOSS but the best ebook manager I've found is Alfa. https://alfaebooks.com/. Source: over 2 years ago
calibre - Ebook manager, viewer & converter
AlReader - Alreader.com - new perspective on reading e-books.
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Quickreader - Quickreader is a book reading application for the leading smartphones that is combining the solution of two main issues of the book readers namely book reader and availability of books from the leading publication houses.