Hello Kovid, thanks for taking a look. I got calibre by running:
sudo -v && wget -nv -O-
https://download.calibre-ebook.com/linux-installer.sh | sudo sh /dev/stdin version=4.23.0
...that is the official way, is it not?
Just to be sure I double checked that there wasn't some other version accidentally in the path:
➜ which calibre
/usr/bin/calibre
➜ ls -l /usr/bin/calibre
lrwxrwxrwx 1 root root 20 Nov 9 08:37 /usr/bin/calibre -> /opt/calibre/calibre*
The squashfs you see in the strace output is just part of the contents of /proc/mounts as it is being read in. Looks like various bits of gnome running out of snaps, seems to be the Ubuntu way now. But nothing directly to do with calibre.