The "History of Middle-Earth Index" is a hardback first edition book written by J.R.R. Tolkien and Christopher Tolkien, published by HarperCollins. This book serves as an index to the entire "History of Middle-Earth" series, making it a valuable resource for fans and scholars of Tolkien's works. With its comprehensive index and inclusion of both authors, this book provides a convenient way to locate specific information and explore the rich and detailed world of Middle-Earth.