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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.
Calibre is available for Linux, OS X, and Windows, 32-bit or 64-bit.
Pro Open Source
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.
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.