The Necronomicon and Rene Descartes

What would the Necronomicon be like to read?

There's a reason I'm asking this question because I think the Necronomicon would be different than the book portrayed in the Lovecraft novels, and many others besides. For those who don't know, this is a fictional book which drives the reader insane. There's more to it than that, but you get the point. Character stumbles across the book and against their better judgment, they start reading. By the end of the page (in the story), the character is jibbering nonsense and foretelling a dark end for all humanity.
It must be one heck of a book!
So this has lead many authors to speculate exactly WHAT is contained on the cursed pages which will put a mind so far around the corner that they no longer connect with a bustling humanity. Some have thought the book contains spells to conjure dark powers, or gives answers to dark secrets better left alone. All of this makes for some great reading.
But what if you came across a book on a dusty shelf in a forgotten library and started flipping through it just to realize with horror that you've founded the blasted Necronomicon and have already set foot on the path which leads to madness. What exactly are you reading.

I've got an idea, but it has nothing to do with spells.

First off, let me tell you a little something about a philosophy class I took many years ago. This was Philosophy 101, essentially a primer for people sort of interested in philosophy but without a firm grasp on exactly what it was. The book the professor decided would make a good primer for students was "The Essays" by Rene Descartes. Good stuff. Not to heavy, not to light. So a good place to begin with studying the nature of thought.
There was one catch however. All Catholic students in the class had to get written permission from their church to read the book. It seemed quaint at the time, but there was a good reason for the caution. The church was acknowledging that students reading the book were in danger of encountering something they might find so upsetting that it would challenge their basic beliefs about God and the world around them.
Now as far as I know, no students came to irreperable harm as a result of their studies so disaster was averted. But this leads to some interesting observations:

1) Reading a book CAN cause irreperable harm to a person's mind and/or sanity
2) The content of the book wouldn't necessarily involve spells or magic
3) The book can be understood in English

So, if a book with the effects of the Necronomican can exist, the question becomes "does it already exist?" I'm going to say no. Right now, a book with the potential harm of the Necronomicon doesn't exist. Sure, I've read some pretty bad books before which have caused me discomfort and distress, but nothing that shook my sanity. I suppose the closest would have been the Dan Brown "Da Vinci Code", but that was just because of the poor writing and not the result of malice on the part of the author. At least, I don't think there was any intentional malice intended.