Modern PHY is like bit twiddling in programming, 99% programmer don't do it and don't care unless you are doing HPC or device driver. In networking those who're doing PHY belong to communication engineering field. For modern networking the relevant PHY mostly dealing with digital I/Q signal for wired/wireless and there are many excellent books on the subject such as this [2].
[1]The Art of Electronics (2015):
https://news.ycombinator.com/item?id=38748370
[2] Digital Signal Processing in Modern Communication Systems
https://www.amazon.com/Digital-Signal-Processing-Communicati...
> "with an early emphasis on application-layer paradigms and application programming interfaces"
I don't agree. For those onboarding onto the subject, their mental model will be focused around the higher level details implemented around the application layer. The physical layer only starts to become remotely relevant once you start to delve onto very specialized topics.
I highly doubt even 1% of engineers could describe how WiFi works on the radio / physical level let alone anything about the variosu ways we make wires and light works.