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I don't remember this being a book, but there is a similar story on the /tg/ archive, "the veil of madness".
I think there is an archived thread whose archived title is "humanity trolls the galaxy", you should check on sup/tg/
.
Well, apparently, "Humanity trolls the galaxy"
is actually a sequel. I Archive Trawled through the Humanity Fuck Yeah threads and found a screenshot of what seems to be the original Veil of Madness
◊.

This is a book that I once read about here in tv tropes. I don't remember the name of the book, but if I remember correctly, It was a sci-fi about humanity living in a part of the galaxy were every other intelligent species turn insane. Humans then expands and find several abandoned or dead planets of the other civilizations than became insane, Including a new agrarian species in the process of killing itself.
When humans finally find some normal, sane, alive aliens, we arrive in the biggest ship in the galaxy, and because the rest of the galaxy is pretty crowded, and we have so much space in our sector from all those dead worlds, we turn to be pretty much the bogeyman of the rest of the galaxy, and humans decide to embrace that notion and play the part to gain an advantage in negotiations.
If anyone knows the name of the book please post it here. I have being dying to remember it.