I have installed Mint 21. I have looked everywhere for Libby as I would like library software. There are people who seem to have installed this on mint but I cannot find how to do this on my laptop
do you mean https://snapcraft.io/libbylinux ?
here are the instructions how to make snap support work on mint:
after following this you should be able to just do:
sudo snap install libbylinux
to get libby installed …