Contents--What is Modern Music--and Why have People Never Liked It, at First?; Music has Always Told How People Think and Act; Dissonance--the Salt and Pepper of Music; Acoustics and the Development of Harmony; Impressionism--Debussy and His Followers; Schoenberg and Atonality; Music Written in Two or More Keys at Once--Polytonality; Back to Bach--Neoclassicism; Music for Everday use--Gebrauchsmusik; From Plain Song to Jazz--A Story of Rhythms; Tone Clusters, Quarter Tones, Percussive and Electronic Music; The Composer and the Public; Selected Reading List; Selected Recordings; Index.
I: "Modern" is a delusion of aspect. Every articulate observer sans time machine is at the end of a chain of events reaching back to antiquity. II: DIssonance is the eventual object in the evolution of composers' imaginations and listeners' ears. III: Enjoy and explore experimental and challenging music.
Not too technical, it actually is a "Popular guide", and in being a popular guide, it succeeds. If you're a composer/music student, you might not find this very useful, but it will be 100% enjoyable anyways.