How does the author structure the book, Unfinished Tales: The Lost Lore of Middle-earth?

The book is divided into four parts, which are named after four Ages in the history of Middle-earth. Each book is divided into more than one chapter. The First Age has two chapters; the Second Age has four chapters; the Third Age holds five chapters; and the Fourth Age contains three chapters. Each chapter is named after its main characters or main plot point. Though the four parts of the book organize the stories overall into a rough chronological order, some of the chapters (such as the ones describing Numenor, the Istari, and the Palantiri) are largely descriptive and are related to happenings over many hundreds or even thousands of years, while others (such as the first two chapters) describe events that were happening more or less simultaneously. Thus, the timelines of many of the stories overlap in many places and meander back and forth.