I think John Emerson is basically right that it has to do with the peculiar sample at LibraryThing in the first place. I’ve basically observed from my own catalog that there are three very strong subsets of LibraryThing cataloguers, with a fourth minor variant.
The three major groupings that I can see are: SF/fantasy, political theory and philosophy (usually leftish), and Christian (whose catalogs are also often documents of their homeschooling projects). The fourth smaller category are academic book collections, though a lot of the academics who catalog there tend to do a couple of hundred books out of what are probably larger collections and stop there. (My own is still missing about 1,500 or more books from downstairs in our house.) There are also some collections which are very heavy on chick-lit and self-help books, I think.
So I’ve found, like John Holbo, if I enter almost any SF/fantasy title in the Unsuggester, almost all of the anti-books are theology and Christian-related. There are some curious exceptions. CJ Cherryh’s Downbelow Station fetches up a real dog’s breakfast of anti-books. Some Christian stuff, but also some chick-lit. The Lion, the Witch and the Wardrobe not unexpectedly turns the mapping on its head: the top two anti-books for it are de Certeau’s Practice of Everyday Life and Richard Rorty’s Contingency, Irony and Solidarity.
You can find some non-genre works that different clusters of LT cataloguers all tend to own where the anti-books are really odd, where it is hard to make a lot of sense about the relationship. Achebe’s Things Fall Apart is the anti-book of works as diverse as Kevin Mitnick’s book on computer security, Anne Rice’s vampire novels, guides to Java programming, the Harry Potter books, and Nora Roberts novels. Marx’s Capital is the anti-book of works of historical fiction, fantasy, Stephen King novels, and The Sisterhood of the Travelling Pants. Works of humor, interestingly enough, tend to be anti-booked by Christian and theological writing just as much as fantasy and SF are. But then, so is Salman Rushdie’s The Satanic Verses.
Like John Emerson, I’m sure this is largely a function of how well walled-off various LibraryThing catalogs are from each other, not necessarily a good description of communities and practices of readership at large in American or global culture.
It’s also a function of the threshold of 75 owners for anti-book recommendations. I’m guessing that there are a lot of titles at LT that have 5-50 owners, that there is a “long tail” of books, some of them pretty notable, that somehow just aren’t heavily represented in the pre-existing clusters of motivated cataloguers.