Created attachment 563140 [details]
Point no 1 and 2
Calibre in every available version has some problems with displaying covers.
1. It show generic Calibre cover in main window instead of the one used in book, cover is even without title and author.
2. Generating cover in calibre does nothing - new cover is not generated, always shows generic one.
3. Obtaining covers also does not work. Log says that it found covers and download them but it does not show them.
For me looks like some missing image processing related dependency since same versions works perfecly for me on other distros.
Created attachment 563142 [details]
Log from built in cover lookup tool
Created attachment 563144 [details]
Spend some time analyzing Calibre code and finally found a solution.
Calibre uses PyQt5 with USE=gui necessary to install. This flag pulls dev-qt/qtgui but Calibre ebuild does not enforce user to set USE=jpeg to dev-qt/qtgui which is needed to display and generate images, because Calibre uses QtImageWriter class and internally operates on jpeg images.
Adding USE=jpeg to dev-qt/qtgui solves the problem, tested on stable and newest version available in official overlay.
Proposed fix: require USE=jpeg for dev-qt/qtgui in Calibre ebuild