To give credit to those sources I've used to create campaign material so far, here are the books I've used in whole, in part, or as inspiration:
Player's Handbook
Dungeon Master's Guide
Monster Manual
Xanathar's Guide to Everything
Mordenkainen's Tome of Foes
Volo's Guide to Monsters
Sword Coast Adventurer's Guide
Ghosts of Saltmarsh
Guildmaster's Guide to Ravnica
Explorer's Guide to Wildemount
Mythic Odysseys of Theros
Tasha's Cauldron of Everything
Plane Shift: Dominaria
SJA4 - Under the Dark Fist
TSR1049 - Adventures In Space
CGR1 - The Complete Spacefarer's Handbook
SJR5 - Rock of Bral
Old Dragon Magazine articles
Strongholds and Followers by MCDM Productions
Faiths of the Forgotten Realms by Pyromanic Press
Arcana of the Ancients by Monte Cook Games
Journey to Ragnorak by Mana Project Studios
Odyssey of the Dragonlords by Modiphius Entertainment
Dungeons of Ice by R&D Adventures
Oath of the Frozen King by Absolute Tabletop
Arcadia by MCDM Productions