This error has been persisting for a few version and is still present in 4.5
Code:
$ ebook-viewer somebook.epub
qt.qpa.plugin: Could not find the Qt platform plugin "cocoa" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.
fish: 'ebook-viewer Robert\ B.\ Cialdi…' terminated by signal SIGABRT (Abort)
Calibre works as expected, and opening ebook-viewer from within Calibre works as expected.