Appendix N receives a great deal of (justifiable) accolades. The list of suggested reading is essential in understanding the rationale behind Dungeons and Dragons. A few pages before that though, page 217 to be specific, is a tool invaluable to dungeon masters of all levels of experience: Appendix I: Dungeon Dressing (Miscellaneous Items and Points of Semi-Interest for Corridors and Unpopulated Areas or to Round Out Otherwise Drab Places).
Aside from displaying some wonderful Gygaxese, this collection of charts and lists forms an increasingly important resource for my dungeon creation, and for my other writing. We have intellect-prompts for air currents, odours, unexplained sounds and weird noises, the furnishings of a torture chamber, and the contents of all sorts of containers, and many more. I’d give these lists priority on my DM screen (if I used one) above the likes of movement rates or equipment lists. They work equally well when preparing a dungeon, or when filling in detail on the fly.