The subject of “favorite books” comes up, someone says (book title) is their favorite book, and the answer causes you to walk away from them. What one book, (no lists PLEASE) would it be?
If there can be only one, then Mein Kampf seems like the easy winner.
To Serve Man.
Hannibal. 
Dianetics
The bible.
Unless they are a scholar in a subject that requires them to do textual analysis of it, and they have become enamored of some of its poetry.
The Book of Mormon
The Bible, but for a different reason. The vast majority of people that claim it is their favorite book have never read it cover-to-cover, but kind of know a precious few lines that support what they already believe. I cannot respect a person that has never read their “favorite” book.
You’re going to basically wind up with a list in this sequence of posts since presumably one can’t mention a book that’s already been mentioned.
Anyway, my nomination is Atlas Shrugged, by Ayn Rand, so beloved by libertarian lunatics and a sort of Bible of libertarian thinking. Among those who have spoken adoringly of this book were Ludwig von Mises, Glenn Beck, and Clarence Thomas.
That is not at all what I meant. I am trying to avoid posts that have a list of books.
The Art of The Deal.
How to Talk to a Liberal (If You Must)
I mean, there are a ton of books that would make me walk away from someone figuratively (“no second date.”) The list that would make me literally walk away as soon as they said it is much shorter, but would include several of the ones listed, as well as The Turner Diaries.
There’s no “one” book. If some adult said “Goodnight, Moon” was their favorite, I’d walk away just as fast as if it were Mein Kampf or The Secret, but for different reason.
Given that, Mein Kampf.
The Satanic Bible.
I wouldn’t assume someone reading Mein Kampf is a Nazi; indeed, there are plenty of good reasons for an anti-Nazi person to be reading it. You can consider a book a favorite even if you oppose it - in a “fascinating in that it shows how bad people think” way.
Don’t start a debate in the thread please.
You haven’t even given your answer to the OP.
You’re welcome to take this to another thread if you like.
Moderating
Chariots of the Gods
Lord of the Flies
Everybody Poops