The OSR changed the way I look at RPG gaming, but I can't point to a single product. The real genius was/is hidden & scattered across the blogs.
Lot of this. Jacquaying the Dungeon and No Quantum Ogres were key for me. Plus the general discussion around keeping all of xp for gold, encumbrance, time management, resource management, wandering monsters, reaction rolls and morale checks, and the game that emerges out of all of those.
But I don't have links to hand.
1E AD&D DMG
Also this.
But all that said, I would include the following in any well stocked OSR bookshelf:
Yoon-Suin
Judges Guild's Ready Ref Sheets
The Caverns of Thracia
Unless you've already started one of your own, one of the newer megadungeons. Barrowmaze or Anomalous Subsurface Environment are strong options. You don't actually need them all though.
One or both of Dyson's Delves books in print. Key at least a couple of the maps to have ready for play.
From ACKS, Lairs & Encounters for some ready to go overland lairs. The core book for the GM chapters in back if you plan to or might get into domain building or merchant trading later, even if you're not running the system.
Veins of the Earth
How to Run, by Alexis Smolensk. Technically old school rather than OSR for those who make the distinction, still a useful look at the high-prep style of GMing.
A False Machine 2011-2014. Patrick Stuart's better blog posts in print. So available for free if you don't mind clicking "older posts" a lot, but something I wish more of the best/influential bloggers had done. Some of those blogs are going dark and the posts lost.
At least one of the arthaus books. A Red and Pleasant Land, Lair of the Blue Medusa, Fire on the Velvet Horizon. Deep Carbon Observatory is arthaus-adjacent but more useful to arthaus-skeptical GMs, so check that out if nothing else.