ARRL Quickstart Guide for All Hams

An quick introduction to Ham radio written by the Amateur Radio Relay League. Introduces key terms and concepts and needed equipment for amateur radio.
NASA

The National Aeronautics and Space Administration provides great resources for learning about amateur radio, including applications into space!
Inspiring Students with Ham Radio
Ham Radio in Space: Engaging with Students Worldwide for 40 Years
History of Radio Broadcasting

A high level summary of the history of radio broadcasting in the United States throughout the twentieth century, including an timeline archived here.