What are some good resources for learning Bluetooth?

I'm noticing a stark decline in the quality of bluetooth products lately (interference, disconnecting, dubious sound quality), and I'm curious if I can figure out what's going on.

Are there any good books for learning about the Bluetooth technology used in generic PS4 controllers? I'm not sure if that's bluetooth LE or classic or 5.0, but that's where I would like to start.

Any books or online resources just to get my foot in the door would be greatly appreciated.