Something screwy is going on with Amazon's sales rankings. And their search engine. Soemthing really screwy with their search engine: I tried a search for Heather Has Two Mommies, and on my first search the eponymous book came back as the 10th result. Books unrelated to it and matching none of the words in the title, plus imitations of it and ones about it, all ranked higher. Buh?
But when I searched again, I got it as the top hit. Buh?
It also has no sales rank. Contrast this with, say, Schlock Mercenary: the Teraport Wars, which has a sales rank of #637,692, or Alysha's Fall, rank #4,559,584. I'm pretty sure it's not that Heather Has Two Mommies hasn't sold enough copies to get a sales rank.
According to Publishers Weekly, Amazon is saying this is a glitch, and not a new policy.
According to Mark Probst , Amazon Support sent him an email that it was an intentional change to delist "adult" material. (And on what planet is Heather Has Two Mommies "adult" material? O_O Daddy's Roommate likewise lost its sales rank). Probst has a link to a Playboy Centerfold special that kept its ranking, but what roaming about I did trying to find heterosexual adult erotica also came up with a bunch of no-sales-rank results. So if it's a glitch, it's a glitch targeted at pretty much anything GLBT and most (but not all) erotica.
My guess is that it was intentional, but either it was intentionally done by someone who didn't really have the authority to do it, and/or Amazon didn't expect a backlash and is now so embarrassed that they're trying to squirm out of it. :6 The "Publisher's Weekly" piece at least suggests that the rankings are going to come back. It's still annoying at best. Sheesh!
Edit: Bunch more links and info here. Also biased towards "it;s an Amazon plot" but has links on Amazon defending itself.