Check out: Introduction to Telephones and Telephone Systems (3rd Ed.) by A. Michael Noll: A great system-level overview covering instruments, transmission medi

Check out: Introduction to Telephones and Telephone Systems (3rd Ed.) by A. Michael Noll: A great system-level overview covering instruments, transmission media, switching, and signaling. Understanding Telephone Electronics by Carr, Winder, & Bigelow: Focuses on the electronic components and workings of telephone systems.