Content (opt-out):

There is lots of gross stuff in the Book of the New Sun! Including:

- Torture and public executions. Severian was raised by the Guild of Torturers and this is his trade up to the point when he becomes Autarch.

- Casual misogyny. (Women are prohibited from becoming professional torturers because women are too cruel, you see. This was decreed by the first Autarch, Ymar the Almost Just. I am not making his name up.)

- Ritual cannibalism. To be (sort of) fair, this is not a common practice and everyone is pretty grossed out by it. Nonetheless it is a thing which Severian is induced to do. More than once.

- A giant bird that eats people and can mimic its prey after consuming it, because a chemical in its brain shares their memories with it.

- That giant bird is called an alzabo, and from its brain juices there is made a drug called the analeptic alzabo. Combine that with aforementioned ritual cannibalism and you get:

- Severian carrying around in his head the memories/personality/etc of a lady he used to know, Thecla who incidentally used to be a "client" of his guild -- i.e., she was tortured.

- Suicide of Thecla. This was a mercy, as Severian gave her a knife to put an end to herself. He got kicked out of the guild for it, sort of.

- So you may want to pretty much opt out of all Thecla-related content if any of that bothers you.

At the same time, I have to say, this all sounds very splatterpunk when I describe it thusly, but the narrative is actually incredibly cerebral and distant. Severian expresses scorn for "algophilists", pain-lovers who derive perverse enjoyment from viewing the mysteries of his guild. The torturers view themselves as professionals and take their work very seriously. No one who enjoys inflicting pain is considered suitable to become a torturer.

There is far more metaphysical rambling than gore in the Book of the New Sun. You may also opt out of metaphysical rambling if you choose.