Amazon Rank

ETA 1 and 3: More here and here.

ETA 2: I should also note that, as a bookstore employee of five or six years now, a book's sales rank is incredibly important. There is a reason the store corrals the bestsellers into a prominent area; people walk in all the time and want to know where they are. The majority of people don't care about books that don't sell well; they won't buy them, and they won't read them. Not ranking certain books hurts the authors. And, of course, removing them from search - so that people who know exactly what book they want still can't buy them - is completely ridiculous.

Note that:
Heather Has Two Mommies,
And Tango Makes Three, and
Uncle Bobby's Wedding
can all still be found at a competitor.