When I described the titles as "unthinkable to the modern sensibility" I was dodging mention of such topics as the "Physiognomy of the Pygmy peoples" or "Universal suffrage by all necessary means" - which are basically books on racism and terrorism, for example as espoused in the works of Emma Goldman.
Old books are dangerous. Ideas are dangerous. Our modern ideas no less so, and to claim otherwise is a "parochialism of the present".
But ones understanding of the world will be much richer, and intellect fortified by at least knowing of these books and ideas, Otherwise we may as well join the ranks of Firemen of Bradbury's 451 brigade.
Other books and authors are just infuriating - the memes they set running weren't necessarily a great thing the first time around, and did a lot of damage to collective human reason. Eliphas Levi immediately springs to mind; there are plenty of others. "Illuminatus!" probably falls into this category, but I don't think that was R.A. Wilson's fault.
[1] https://www.bbc.com/mediacentre/2021/radio-4-jon-ronson-thin...
It's important to recognise that until the internet made it much easier for people to discuss ideas and find counterarguments, there were large numbers of people floating around claiming that the contents of these books were not only beyond reproach, but that <insert group invented in the 20th century> was an unbroken descendant of one of the mythical groups invented by Victorian pseudohistorians.