A Dictionary of Tolkien is an illustrated A–Z reference guide to the people, places, creatures, battles, plants, and events found throughout The Hobbit, The Lord of the Rings, and The Silmarillion. Rather than telling a single story, it serves as a companion book, allowing readers to quickly look up characters like Gandalf or Galadriel, locations such as Rivendell or Mordor, and many other aspects of Middle-earth. The entries are accompanied by illustrations and maps, making it enjoyable to browse as well as use for reference.
One important note is that the book is not an official Tolkien reference and was not authorized by the Tolkien Estate. While many readers enjoy its artwork and accessible format, Tolkien scholars and longtime fans have criticized some of David Day's interpretations for mixing his own theories with Tolkien's writings, so it is best viewed as an introductory companion rather than a definitive scholarly source.
Overall, A Dictionary of Tolkien is an attractive and engaging guide for readers who want a quick overview of Middle-earth's rich history and characters, especially while reading Tolkien's novels. If you're looking for the most authoritative reference, many Tolkien enthusiasts instead recommend Robert Foster's The Complete Guide to Middle-earth.