Anything by Pagan Publishing, really. Their Delta Green books are chock-full of amazing detailed info that will spark scenario ideas right and left. The actuals scenarios in them are very good, as well. Don't try to buy them from most eBay sites or such because people charge an arm and a leg for even the non-out-of-print stuff: get them from
Pagan Publishing: Home
Yeah, I got the PDF for the upcoming Delta Green supplement last week.

(I donated to the Fundable drive.)
Nom, nom, nom, nom, nom....
Sadly, much Pagan Publishing material is out of print, and auctions can be the only route.
I remember the shock on the players faces in Walker in the wastes - the characters lost a battle (badly), then woke up in the hospital, finding out that the cultists had brought them there rather than leaving them to die. (They came up with the theory that something had laid eggs in them....

)
The very, very short adventure Grace Under Pressure is another gem, now out of print.
They even have an entire campaign in which there is no Cthulhu Mythos involvement at all.
I have never been disappointed by them.
The Auld Grump