I hope to see, the Players Handbook as the only book someone needs to buy to play a game of D&D.
Players Handbook = DM and Hero = Rules Compendium, Rules Cyclopedia, Core Rulebook
Dungeon Masters Guide = story advice, 5e Advanced D&D game-changing options, Unearthed Arcana
Monster Manual of Greyhawk
Monster Manual of Nerath
Monster Manual of Feywild
Monster Manual of Eberron
Monster Manual of Ravenloft
Monster Manual of Planescape
Monster Manual of Forgotten Realms
Monster Manual of Dark Sun
Monster Manual of Dragonlance
...
Greyhawk Setting Guide
Nerath Setting Guide
Feywild Setting Guide
Eberron Setting Guide
...
Drow Menzoberranzan Region for Forgotten Realms Setting Guide
Githyanks and Githzerai Regions for Planescape Setting Guide
...
Treasury = magic items, special equipment, vehicles, mounts, animals, castle construction, services, hirelings, ...