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Calibre is open source and even has a portable version available for download (so porting to portableapps is probably not difficult). Any chance we can get this on portableaps?
There's a test build available here: https://portableapps.com/node/20518
Additionally, we do plan to release Calibre officially, however this has not yet occurred.
I think it is already official in the Apps page.
Yes, since this request was made, Calibre has now been released officially.
Cool, that didn't take long!