A lot of older text books put modern versions to shame in many respects.
I have a two-volume set "Telephony," published in 1948. They were considered for many years (decades) the definitive works on the technicalities of the British telephone system. In fact I only just recently acquired my own copies and had to pay £32 ($48) to get them! I have a similar volume which I've had for years which covers the British telex system.
I don't think there are any books to rival them these days for their in-depth coverage of audio transmission, relay design, etc.