At what point does a book become "not worth the paper it's printed on?"?

My husband said this about one of his college textbooks, but I'm sure that it's just because he disagrees with the majority of its contents. To others, the content might be quite meaningful. So can any book ever be "not worth the paper its printed on" completely? Just because some may not agree with what's written, even if a great majority of people disagree with what's written (think Mein Kampf by Adolf Hitler), does that make the book worthless? Or is all literature worth something at least to someone somewhere?