gnome-books was split from gnome-documents, and we need to provide either a stand-alone package, or obsolete the subpackage so that gnome-documents can be upgraded on its own. As the gnome-books upstream maintainer, I already maintain gnome-books on Flathub, and don't plan to offer it in Fedora. If somebody else wants to maintain it, they're welcome to.
Reassigning to correct component, and closing as the changes have been merged: https://src.fedoraproject.org/rpms/fedora-obsolete-packages/pull-request/14