From the back of the book:
HârnWorld is a detailed, realistic, flexible, and system-neutral setting for fantasy role-playing games. Since 1983, HârnWorld has been used by thousands of people across the world to run a wide array of medieval fantasy adventures. HârnWorld can accommodate whatever style of campaign you are looking for, whether your player characters are knights, mages, barbarians, clerics, noble lords, mercenaries, gladiators, craftsmen, merchants, thieves, or even simple peasants yearning for adventure.
Hârn: The 80-page Hârn article is a general overview of Hârnic cultures, governments, economics, history, and more. It also includes campaign aids for such tasks as generating weather conditions and character backgrounds.
Hârndex: Hârndex is a 148-page index with entries for kingdoms and major settlements, historical and p[resent day monarchs and important people, religious and economic organizations, geographic features, and some strange, dangerous creatures unique to the HârnWorld setting. Hârndex also includes definitions of terms related to medieval economics, warfare, and society.
Lythia: The 32-page Lythia article describes the civilizations, religions, and trade of the Lythian continent, as well as the languages and scripts used by its denizens. It includes maps of political/cultural boundaries, topography and vegetation, trade routes and major exports, and language distribution.
Kethira: The 10-page Kethira article includes maps and descriptions of the planet's vegetation, winds, ocean currents, and tectonics. Also included are details of the Nolomar solar system, including charts of the stars and constellations of Kethira's northern and southern skies.
Poster Maps: The maps of the Hârnic Isles and Lythian continent provide a vast amount of information with a unique cartographic system developed specifically for fantasy gaming. Colors and textures show vegetation and terrain as noted on the map keys.