The Prologue (The Peoples of Middle-earth)

'The Prologue' is the name of the first chapter of The Peoples of Middle-earth - the twelfth volume of The History of Middle-earth.' 

The chapter that follows is "The Appendix on Languages".

Chapter synopsis
Christopher Tolkien lays out the typescript history of his father's Prologue and Foreword to The Lord of the Rings, the oldest typescript of which originated in 1938-9. He explains his father's various alterations to the written lore of Hobbits and the Shire, accounting for words and names that appeared in one edition of the Prologue but not in the next.

The earlier version of Riddles in the Dark, the tale of Bilbo's encounter with Gollum told in The Hobbit, is also featured.

Reference

 * The History of Middle-earth, Vol. XII: The Peoples of Middle-earth, Part One: The Prologue and Appendices to The Lord of the Rings, chapter I