Howstuffworks.com -- general outstanding source of technical information about telecommunications (and other things) explained in lay terms. Highly recommended.
David Cyganski and John A. Orr, Information Technology: Inside and Outside (Prentice Hall 2001). An excellent, readable textbook.
Ray Horak, Communications Systems & Networks (Mark A. Miller consulting ed., 1997).
Annabel Z. Dodd, The Essential Guide to Telecommunications (1998).
Gilbert Held, Understanding Data Communications (5th ed. 1996).
Advanced
E. Bryan Carne, Telecommunications Primer: Signals, Building Blocks and Networks (1995).
Arthur Lemuel Albert, The Electrical Fundamentals of Communication (1942). Although an older text, Chapter 13 of Albert’s book explains clearly the transmission of electromagnetic waves down copper wire. The explanation is clear and concise.
Nathan J. Muller, Mobile Communications Factbook (1998). This book covers the gamut of communications technologies. Useful as a reference as it includes descriptions of all major communications technologies and protocols.
Propagation of Radio Waves (M. P. M. Hall et al. eds., 1996). This work is highly technical. However, the first chapter sets out the electromagnetic spectrum with a general description of properties and vocabulary.
Cotter W. Sayre, The Complete RF Technician’s Handbook (2nd ed.1998). Somewhat off the beaten path, this technical reference contains excellent explanations of modulation techniques, as well as their advantages and disadvantages.