He refers to different periods of pop-rock music history.
1. "Book of Love" by Phil Spector
2. Buddy Holly, Big Bopper and Richie Valens (In those days they didn't drug but drank)
3. Bob Dylan (about the clown part)
4. Altamont (mentions Jumping Jack Flash and anger and Stones)
5. Nostalgia for how it was so nice and simple and uncomplicated then it
      got crazy in the Sixties and Seventies.
Each stanza contains references to how rock progressed.

Of course Don doesn't want to give away the store. Otherwise would you listen to it and try to figure it out?