But here's one I wasn't aware of. I know I read The Egyptian Cross Mystery by Ellery Queen at some point. I've read all of the real Queen novels (and some of the others, the ones that were actually ghostwritten or otherwise outsourced), though for some reason I don't own a copy of this one.
But I recently read something about it online, and now it's obvious where I got a key plot element in "The Church Murder Case" (I'm not providing any links – you'll have to get your own spoilers if you want them).
But now I've ordered a copy, since I'm eager to find out if I borrowed any other elements as well. I do know that the motivation in "The Church Murder Case" is one that Queen used at least once, but I have no idea if it was used in this particular book.
Damn, this is like finding a whole new EQ book that I never read before.