I'm in the hold pool of an A320 operator and I've recently started looking into it. Plenty of nice resources on the internet. SmartCockpit is a good place to start and
Airbusdriver.net is another site that is being kept up to date.
Then you have
Airbus A320: An Advanced Systems Guide and
Thea320studyguide.com. To practice ECAM handling there is
www.usebeforeflight.com. And there is also
https://v-prep.com/.
But at the beginning, you're better off sticking with your operator FCOM.
EDIT
Also Airbus official publications are a good source for Airbus pilots.
https://www.airbus.com/aircraft/supp...t-library.html
https://www.airbus-win.com/