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For me, it would be Whispers and the Roars by K. Webster
For the first half of the book, I was waiting patiently for things to go downhill.
And when it did, I had not expected what happened. I had a little doubt, but no idea about the intensity of the matter. I had to stop reading and stand and walk around after every 15 minutes during the other half of the book.
I dont even know if I should suggest you read it. DON'T read it.
Well if you are acquainted with abnormal dark romances with erotica maybe...read it.

The most disturbing book for me was Empire of Storms (Throne of Glass series) by Sarah J. Maas. The reason it was so disturbing, that something terrible happened with the main hero at the end of the book, which included a lot of violence. I was so disturbed by this book that I could not read the next books in this series, though I like this author.

The most disturbing book I read was Nullform by Dem Mikhailov. Cannibalism is a big topic.

I found the ending of Thinner by Stephen King very disturbing

One of the most disturbing books I read in recent time is by German author Harald Welzer: Täter. Wie aus ganz normalen Menschen Massenmörder werden

I'm not sure if there is an English translation, I read it in Dutch. He is investigating how normal people can become mass murderers. This is a Google Translation from the German description of the book:

It shows in a disturbing way how willingness to kill is created and how little we can trust our moral convictions. A lot has been written about the Holocaust, but the most important question has not yet been answered: what were all the "normal men", good-natured family men, like? and harmless average people capable of killing people en masse? There was no group of people who refused to accept the call to murder, which is why explanations that focus on the personalities of the perpetrators, their character traits, and their psychological state are of no use. Welzer examines acts from the Holocaust and other genocides in their social and situational aspects framework and shows how killing can become, within a few weeks, a job that is done like any other. His social psychological study opens up a perspective on the perpetrators that sheds disturbing light on how the willingness to kill is created and how little trust we should have in the stability of our moral convictions.

This was recently discussed in one of my Facebook groups and many people said Pretty Girls. I have that one saved to read. So far, I would say If You Tell

The most disturbing book I've read was House of Salt and Sorrows by Erin A. Craig. By the time I realized the book wasn't going to be less dark as it went on, I decided I needed to finish it so that I would have some resolution and not get as freaked out as having the ending open ended.

I think The Clockwork Orange is up there. It's disturbing both in theme and content.