I have a question that I am going to ask myself and I hope my readers will ask it of themselves and share their answers in my comments section. I’ve been playing D&D for 30+ years. I’ve got a fairly extensive Dungeons and Dragons library. As I build my world and create my campaigns, I make use of that library. Now the question, “What resources from my collection do I use the most and why?”
Dungeon Masters Guide (1E) – Tables – There are tables for nearly everything I could want: Gem Values and Magical Properties, Expert Hirelings with explanations of what each does and their Monthly Costs, and Powers and Side Effects for Artifacts and Relics to just name a few.
Monster Manual (1E) – Illustrations – I’ve seldom seen better illustrations of the monsters I use in my games. No offense is intended to any of the many great artists who have illustrated numerous D&D products, but sometimes a clean lined black and white illustration sparks the imagination the best.
Fiend Folio (1E) – Slaadi
Dragons (1E Role Aids) – DRAGONS! This is a great setting and resource book. I’ve used it for treasures, NPCs, and settings.
Denizens of Avadnu (3E Setting) – Great set of monsters in an non-standard D&D setting.
Gods, Demigods, and Heroes
Deities and Demigods Cyclopedia (1E) – Pantheons – I just enjoy looking through book and finding a pantheon or a god that would add a unique flavor to a region or an NPC.
Deities and Demigods (3E) – Advice on building religions – The examples of mystery cults and monotheistic versus polytheistic religions are good reads, useful and fun.
Religion (GURPS) – Title says it all.
Dragon Tree Spell Book (1E The Dragon Tree) Spells – Wild, weird spells from the early days of gaming.
Psionic Artifacts of Athas (2E) – Magic items, psionic tools, and life-shaped items. I’ve made extensive use of the Rhul-tal.
Sorcerer’s Guide (Talislanta) – Magical Tomes, Magical Items, and Extra-Dimensional Entities
Tome of Mighty Magic (1E North Pole Press) Spells – More, wild, weird spells from the early days of gaming.
Windriders of the Jagged Cliffs (2E) – Life-shaped items and language
Domains of Dread (2E Ravenloft) – Great NPC ideas and adventure sites
Heroes of the Feywild (4E) – NPC ideas and site details
Heroes of the Shadowfell (4E) – NPC ideas and site details
Hordes of the Abyss (3E) – Good ideas for demonic and dark extra planar sites
Manual of the Planes (1E) – Good ideas for extra planar sites
Manual of the Planes (3E) – More good ideas for extra planar sites
I am sure there are others, but these are the ones that I have been reaching for most, when I am working on my new 5E game.