For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:michals:Calibre/openSUSE_Tumbleweed/home:michals:Calibre.repo
zypper refresh
zypper install calibre
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:michals:Calibre/15.6/home:michals:Calibre.repo
zypper refresh
zypper install calibre
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:michals:Calibre/15.5/home:michals:Calibre.repo
zypper refresh
zypper install calibre