Some kind of weirdness has been going on at Amazon. Apparently involving books with "adult" themes being removed from sales rankings and with much of that seeming to overly focus on books with gay themes or content. Or something. There's a petition and Amazon certainly seems to be deserving of the negative attention they're getting for this. Any kind of "adult" filtering that messes with sales ranks or otherwise hides information doesn't seem like a good thing. Content filtering that targets Virtually Normal or Running With Scissors or Hothead Paisan or Tipping the Velvet or the Celluloid Closet (all w/o sales rank info as of a few minutes ago)... well, that's when it's time to think about taking my business elsewhere.
Curiously, Brokeback Mountain, And the Band Played On, Howl and Other Poems, and Take It Like a Man (Boy George's autobiography) all have sales rankings at this time.
NOTE: I've seen several mentions online that Amazon is claiming this was a glitch but I can't find any original source documents to confirm that.