Now through Saturday my book of short stories The Carnival Papers is free for Kindle. At the moment it's #2 in the Short Stories category (free, not paid of course). In case you don't believe me:
Another example is a book I published under a pseudonym. It's done nothing in America but it's sold about 100 copies in the UK in the last few months. The reason is it got on the bestseller list for horror short stories (paid this time!). It's been in the 30s though at some points even higher. Often it's beating out something by Stephen King, so that's something.
I think with that one it started again with a sale through KDP Select which led to some sales, which led to getting on the bestseller list, which has led to more people seeing it and buying it.
So the tip is if you're book is in KDP Select, it's good to use your free sale days when you can because that can drive sales. It's like my marketing professor in college said: People can't buy your product if they don't know it exists. If you can get on a bestseller category then it helps people know your book exists. It's been a lot more effective for me than Tweeting and blogging, I tell you what.