Clif,
The 5th of a chord is the least important note in determining a chord's color and is usually the first note to eliminate when you run out of fingers (haha) and want to include chord extension tone colors like the 9th, 11th, 13th, etc. Then we get into altered extensions like b9; #11, b13. But that is covered in a later chapter

The example of 5ths that Uncle Dave is referring to relates to the "Cycle of Fifths". Check out the following Cycle of 5ths Chart: http://visionmusic.com/keyscycle.html

Scott