In a somewhat different display of how luck can help out an author, Mary McDonald had her No Good Deed made free by Amazon. This isn't entirely up to luck, admittedly, but luck does play a role, because you can't make a book free on Amazon. The best you can do is offer it for free somewhere else, and hope Amazon will price match (they don't always choose to do so, which is where the luck comes in). But it worked for Mary, and her book shot way up the free rankings. When it switched back to 99 cents, it continued to do well, remaining in the top hundred for two weeks (it's still ranked at #125). While No Good Deed has been a perennial seller, this seems to be the best it's ever sold.
Meanwhile, Victorine Lieske's sophomore effort, The Overtaking, continues to be ranked fairly high-- right now it's around #38,000. I'm sure it will eventually take off as Lieske's first book did, but this is a good example of how strange indie publishing can be. Lieske's first book sold literally tens of thousands of books a month, getting her onto the New York Times Bestseller list, and is still ranked at around #600. It's hard to imagine why all those readers aren't snapping up her second book instantly. But for whatever reason, it's taking a while to get rolling.
This all makes me nervous about my next book. Luck can play such a large part in sales, and there are so many variables that no one seems to understand!