Seem to be like an easy fix.
Current calibre ebuild does not list Qtwebkit as dependency. Is this report about a missing dependency (as compared to missing use dep) or a version that has since left Portage?
The Qtwebkit dependency is indirect, via dev-python/PyQt5[webkit].
Since PyQt5 doesn't have an icu USE flag, we'll have to add a direct dependency on dev-qt/qtwebkit.
Upstream has a qtwebkit dependency listed here: