I've got no dog in this fight, as I'm not currently running an OSR game, and I'm not in any way connected with this campaign, but this Kickstarter monster book leapt out at me as an incredibly usable work for OSR DMs:
The Monster Overhaul.
I'm at the stage of my DMing career -- in the neighborhood of 40 years, yikes -- where I want things to be immediately useful at the table. (I have inspiration for days at this point, thanks.) A book that instantly gives me a list of names for NPCs when my players would rather talk than fight, a random table of colorful descriptions, other things that answer the needs of an actual DM playing the actual game? Yes, please.
Too many books seem to have never been playtested and, moreover, have little to no connection to how they will be used in actual play. This is the exact opposite of that, and I love it. It tempts me to
pick up Advanced Fantasy OSE go back to Castles & Crusades just to use it.
(Skerples also created
Magical Industrial Revolution, about rapid "technological" magical innovation changing the world -- and almost certainly dooming it. It's a great fantasy city that makes preventing multiple apocalypses fun and even funny. If I didn't already have an ongoing Ptolus campaign, I'd start a campaign in Endon tomorrow. It's also designed to be incredibly useful at the table, so this is just a thing Skerples is into. God bless.)