Work your way through the Airbus Safety First magazine series. They're free and while they do include a lot of systems stuff, it's all practical information about how things work.
The pneumatics section of the Airbus FCOM is particularly impenetrable though on first reading. It's not helped because Airbus FCOMs are designed to be read by non-English speakers and as such are artificially limited in terms of the language and vocabulary used. I believe Airbus have a list of words they're allowed to use in the FCOM, so if the one word that explains something obviously isn't there, they have to pick another combination.