Amazon ranks are notoriously unstable and constantly in flux. From what I understand, they provide an indication of
current sales.
The number of reviews, I've found, can serve as an (admittedly) rough and imperfect indicator of how well the book has sold throughout its lifetime.
To illustrate my point. Harry Potter and the Sorcerer's Stone currently has a rank of about 31,000. It also has over 5,000 reviews.
http://www.amazon.com/dp/054506967X/?tag=absolutewritedm-20
Another book, titled Forever My Lady, reviewed by Publisher's Weekly, with 52 amazon reviews... Has an amazon sales rank of over 1,000,000.
http://www.amazon.com/dp/0446545562/?tag=absolutewritedm-20
Here's a quote from the author's website. "Forever My Lady was featured in national magazines, on national television and radio including: The Boston Globe, Miami Herald, Rotarian Magazine, Urban Latino Magazine, VOY, Right On! Magazine, NPR, American Latino TV, SITV and was awarded Best New Author Award and the Best Urban Fiction Award and by the Mahogany Media Review."
Going down a list here for Brown's...
10, 38, 3, 1, 5, 10, 6, 6, 4, 2, 3, 4, 3, 26, 5, 0, 15, 20...
Like I said, it's not random house, but it might not be a bad alternative, if people know what they're getting into. Obviously, do your research, ask for a list of the bestselling books if you're offered a contract, ask what the median sales figures are, etc..