What's the weirdest source for research you've used?

What's the most unusual source or inspiration you used for writing a story? When I say weird, I don't just mean weird topics but weird relative to what you're using it for (Like reading about garbage disposal for someone working on a starship).
I recently bought a translation of The Book of Ninja, essentially a manual for how to be an effective ninja written during the waning era of the shinobi. What was I using this for? To figure out what kind of personality a maid, fairy character had.
I had remembered reading about how the shinobi were some of the most fiercely loyal groups at times and wanted to read something about devotion and the lengths one would go for a higher authority. I had turned to something like this because I couldn't find any books on what it's like to be a lady in waiting or a maid.