How does the author structure the nonfiction book, The Heavens and the Earth?

The book is divided into six smaller books. Each book has its own theme and its own section of history to cover, and does so with a number of chapters devoted to different topics. Each of the books has a titled section with the book number and a conclusion section, each of which are separate from the chapters. This makes the structure of the book a little unusual, though it works as a whole for this particular subject. The book is for the most part chronological, and the author is quite thorough in all relaying of information.