The 1979 Advanced Dungeons & Dragons (AD&D) Dungeon Masters Guide (DMG), authored by Gary Gygax, is often considered the "holy grail" of tabletop RPG rulebooks. While the cover date says 1979, it was the final piece of the "core three" books released to complete the transition from the original 1974 rules to the more structured AD&D system.
The 1979 edition (particularly the early printings with the Efreeti cover by David C. Sutherland III) is a major collector's item.