Teratic Tome
Ford's Faeries
Rifts World Book 1 - Vampire Kingdom - Original Printing as it has the stuff for Circus and Traveling Shows
The Original Ultimate Toolbox for GM by Ibach, Ibach, and Pinto
Cites by Midkemia Press
Tome of Adventure Design Revised
The Arduin Trilogy
Hex Flower Cookbook
Kellri's City Block https://kellri.blogspot.com/
Kellri's CDD1-4 https://kellri.blogspot.com/
Justin's House Rules https://thealexandrian.net/wordpress/7842/roleplaying-games/justins-house-rules-for-odd
Gary Gygax's House Rules https://cyclopeatron.blogspot.com/2010/03/gary-gygaxs-whitebox-od-house-rules.html
Malevolent and Benign by Expedious Retreat Press
Petty Gods Revised
Jason Vey's Conan for OD&D 0 Free Game https://www.grey-elf.com/
Fantastic Medieval Campaigns by Marcia B
Varlets and Vermin
Zenopus's One Hit Monsters List https://zenopusarchives.blogspot.com/2013/02/one-hit-point-monsters_23.html
The Alternate Wandering Monsters List https://beyondtheblackgate.blogspot.com/2012/08/alternate-bx-s-wandering-monster-tables.html
The Majestic Wilderlands by Robert S Conley
Supplement X Philotomy's Musings
The Black Sword Hack Ultimate Edition by The Merry Mushmen
Worlds Without Number - Sine Nomine Publishing
Lavender Hack by Ziapelta Games
For Gold & Glory by God Emperor Games
OSRIC by Stuart Marshall
Swords & Wizardry by Finch - see new revised edition
Supplement v Carcosa by Geoffrey Mckinney
The Perrin Convention - get the pdf of All the World's Monsters II it is inside
of course Chainmail 3rd Edition and it's first 5 supplements - Men & Magic, Monsters & Treasure, The Underworld & Wilderness Adventures, Greyhawk, and Blackmoor
Please Post Up Your List
This list is awesome, and a number of the things on it are also things I keep in my collection. I'll just list the ones that I didn't see here that I use often. The comparison to "official" D&D books is just to state how I typically use them
Splat books would include:
I don't use them all for every game, but I love having them all on my shelf for anything I might need during prep.
Also, a shoutout to the OSE Classic Fantasy Rules Tome. It's so nice, and even if I'm not playing it, there's a great selection of magic items and other random stuff for the DM, and like...it's a really nice looking book.
I've been eyeing some of the Lost Pages books for a while, looks like I need to add The Nocturnal Table to my wishlist - I struggle with city adventures