When comparing FBReader vs Calibre, the Slant community recommends Calibre for most people. In the question“What are the best desktop readers?” Calibre is ranked 1st while FBReader is ranked 6th. The most important reason people chose Calibre is:
Features include metadata editing and downloading, coverpage downloading., book tagging and categorization, webpage-to-ebook converter for popular websites, etc. Lots of plugins available to extend functionality.
Specs
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Pros
Pro Built in eBook store
FBReader features an in built eBook store for users to download eBooks onto their devices through the app.

Pro Simple and fast
FBReader, while offering many options is simple in design, with that allowing for fast use.
Pro Books can be saved to SD card
Pro Cloud sync
FBReader has built in Google Drive sync for the users files, allowing them to store their eBooks online, not taking up room on their local storage.
Pro Powerful book management software, very customizable and scriptable
Features include metadata editing and downloading, coverpage downloading., book tagging and categorization, webpage-to-ebook converter for popular websites, etc. Lots of plugins available to extend functionality.
Pro Cross-platform
Calibre is available for Linux, OS X, and Windows, 32-bit or 64-bit.
Pro Open Source
Pro Portable
Calibre has an optional portable build that can be stored on external media along with the users library. It can then be opened from the external media on any windows device.
Cons
Con Cannot search for books on a SD card
Con No light mode
Unusable if you have astigmatism.
Con UI is bad
User interface is confusing and ugly.
Con Complex and confusing app settings
Con Average, non-power users might find the interface somewhat daunting
The interface, while functional is not the most attractive or intuitive which may put some people off and discourage their use.
Con Can't save text highlights or annotate
