The Peoples of Middle-earth traces the evolution of the Appendices to The Lord of The Rings, which provide a comprehensive historical structure of the Second and Third Ages, including Calendars, Hobbit genealogies and the Westron language. The book concludes with two unique abandoned stories: The New Shadow, set in Gondor during the Fourth Age, and the tale of Tal-elmar, in which the coming of the dreaded Numenorean ships is seen through the eyes of men of Middle-earth in the Dark Years.

With the publication of this book, the long history of J.R.R. Tolkien's creation is completed and the enigmatic state of his work can be understood.