I’ve only learned about this through my engineering studies, so I can only really recommend textbooks. I found Proakis’ Digital Communication quite good, but it doesn’t go very deep.

I don’t know of any online resources – but I’d love to have the time to write some signal processing for communication is fascinating and has a wide impact on everyday life.

Also wrt. implementations: I think GNU Radio might be a good place to look, but honestly the actual implementation of these algorithms is often very simple, it is the theory behind them that gets hairy.

You can lurk Bluetooth specification and everything around, for example.