As I said in my original post;
"Free books don't stay free for long."
So, if you spot something that you think might be interesting, get it. Just make sure the price is $0.00 before you hit the BUY NOW button. But don't worry too much, Amazon is very quick about crediting your account if you make a mistake.
The Bestseller List on the Kindle eBooks page lists the bestselling 100 free books and is a good place to start looking.