So by way of andrewducker's Interesting Links for 07-06-2022, in particular this one, we find the Poison Book Project. See also "How we discovered three poisonous books in our university library" and "This article on the project" on the International Institute for Conservation of Historic and Artistic Works website.

It seems that the the Victorians were fond of bright colors, and in particular the vivid green, arsenic-based pigment known as Paris green. They also used it in wallpaper -- apparently death by wallpaper was a known thing, but that apparently didn't keep the wallpaper industry from using it and dismissing the warnings. More recent parallels include uranium glassware, radium watchdials, and of course the tobacco and fossil-fuel industries.

...And the next time you want to write a poison pen letter, see if you can find some Victorian wallpaper to write it on.