Couple things to keep in mind:
Free books kind of come and go. Right now seems to be a bad time for them --perhaps
amazon's trying to get people to spend their gift cards first? Look at the 100 best selling free books every week or so. You'll find some good things when you're not expecting them.
When you find something good, get it right away. Often authors put their things up for free to get themselves on the bestsellers' list, and then a couple days lat they start charging.