These four books are the supplements to the original Dungeons & Dragons boxed set. Each book expands the game with additional rules:
Supplement I: Greyhawk adds the thief and paladin classes, new combat rules (removing the dependency on Chainmail), spells (including levels 7 to 9), monsters (including lizard men, beholders, displacer beasts, blink dogs, carrion crawlers) and treasure. Greyhawk was Gary Gygax's own campaign, although little setting information is included.
Supplement II: Blackmoor adds the assassin and monk classes, hit location rules, underwater adventuring, monsters, treasure, and The Temple of the Frog adventure. Blackmoor was Dave Arneson's own campaign, although little setting information is included.
Supplement III: Eldritch Wizardry adds the druid class, psionics, demons (including Orcus and Demogorgon), psionic monsters (including mind flayers), and artifacts.
Supplement IV: Gods, Demi-Gods & Heroes adapts various pantheons to D&D, including Egyptian, Hindu, Greek, Celtic, Norse, Finnish, Aztec, Mayan, Chinese, and Japanese mythologies, as well as Robert E. Howard's Hyborea and Michael Moorcock's Melnibonéan Mythos.