I'd observe a difference in the debuginfo data for the plugins in rebuilds. Various /usr/lib64/calibre/calibre/plugins/*.so files would be different, but the source of the difference was in the link to debuginfo data, which in turn appears to be caused by differences in sort order in various tables, caused by the object files being listed in different order. setup/build.py writes out pyproject.toml, which is turned into *.pro, which is turned into a Makefile. Sorting the object files by name seems to fix this particular issue. This makes local rebuilds reproducible for me.
calibre

calibre is an e-book manager. It can view, convert, edit and catalog e-books in all of the major e-book formats. It can also talk to e-book reader devices. It can go out to the internet and fetch metadata for your books. It can download newspapers and convert them into e-books for convenient reading. It is cross platform, running on Linux, Windows and macOS.
For more information, see the calibre About page.
Screenshots
Usage
See the User Manual.
Development
Setting up a development environment for calibre.
A tarball of the source code for the current calibre release.
Bugs
Bug reports and feature requests should be made in the calibre bug tracker at Launchpad. GitHub is only used for code hosting and pull requests.
Support calibre
calibre is a result of the efforts of many volunteers from all over the world. If you find it useful, please consider contributing to support its development. Donate to support calibre development.
Building calibre binaries
See Build instructions for instructions on how to build the calibre binaries and installers for all the platforms calibre supports.