This quote is from the introduction explaining how the book got its title:
“In discussions of global energy and resources with our friends and colleagues, we found that many of them shared our frustration with all of the different units being used to describe energy. What we needed was a large unit of energy that could be visualized and would also evoke a visceral reaction… We turned to a unit that one of the authors, Hew Crane, had devised as he sat in the long lines that typified the energy crisis if 1974. He had heard that the world was using oil at the rate of 23,000 gallons a second and began wondering how much it would be in a year. A few multiplications later, he calculated it to be approaching a trillion gallons. 724 billion gallons to be more precise.”
This happens to be the volume of a cubic mile. So in 1974 we consumed a cubic mile of oil per year. Later in the book he points out that we are now consuming over 3 cubic miles of oil a year 41 years later.