The first harcover book of character races, classes, equipment, and spells for Advanced Dungeons and Dragons. One of the original, core AD&D rule books.
A later volume, Unearthed Arcana, would add some character options and spells, but it did not compare to the solid foundation of the PH. I feel safe in asserting that AD&D would not be the same game without the standard PH character races of dwarf, elf, human, gnome, halfling, and even half-orc. Likewise, the fourfold class structure of fighter, thief, cleric, and magic-user is essential, oddities such as the monk and psionicist classes notwithstanding.