For standard run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mwilck:Leap:15.4/standard/home:mwilck:Leap:15.4.repo
zypper refresh
zypper install calibre
For ARM-15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mwilck:Leap:15.4/ARM-15.5/home:mwilck:Leap:15.4.repo
zypper refresh
zypper install calibre
For ARM run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mwilck:Leap:15.4/ARM/home:mwilck:Leap:15.4.repo
zypper refresh
zypper install calibre