This is the book used by GuitarPrinciples as part of the Mel Bay Study Group. It is used to to begin Stage Two of Guitar Development:
After you have begun the process of learning how to practice by using The Principles, and you have learned the first position chords in the key of G with "The Path Level One: Chords & Rhythm",, it is time for some Horizontal Growth by learning a good number of songs, and some Vertical Growth, learning new first position chords, as well as more sophisticated strums, and accompaniment techniques such as BASS RUNS, and ARPEGGIOS.
You can do all of these quite nicely with this wonderfully laid out collection of folk songs. It goes systematically, chapter by chapter, through the common keys, showing you how to do bass runs between the chord changes. A bass run is simply a series of single notes taken from the scale of the key you are in, and played in place of some of the strums. Along with using Alternate Basses in your accompaniment, bass runs add an entirely new dimension to your playing, and learning how to use them lifts you above the level of someone who can only strum chords.
The book goes through major keys, then minor keys, with lots of songs illustrating the techniques. The system of placing the string number over the fret number is used to denote the bass notes for you (6/4 means sixth string, 4th fret, or G#). This means you can learn this material without reading music.
The Left Hand Foundation Exercises from The Principles will give you the necessary left hand training you need to do bass runs well. Later in the book, various arpeggio patterns are given (also known as "travis picking" patterns". The Finger Dipping exercises from The Principles should be used here as the basis of your right hand technique.
The songs include perennial favorites of mine (I like cowboy songs!) Red River Valley, Streets of Laredo, and "Beautiful Dreamer", the classic by the first great American songwriter, Stephen Foster.