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
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
PHB: Whitehack (4e just came out but it looks largely the same as 3e with a few changes to, I think, excise the OGL)
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