Disco, silly as it can be, is often deeply poignant. It’s an ideal vehicle for catharsis; over a suitably irresistible beat, sadness becomes triumph, and anger becomes vindication. This chapter opens hot with Heatwave and keeps the ardor up throughout, as George McCrae and Candi Staton infuse steady grooves with a lush yearning. I had to end with Loleatta Holloway’s quaking but firmly controlled breakdown on “Dreamin’,” because nothing can possibly follow it.