Since there are so many free books on Amazon's site, I usually go to the section that allows one to browse the bestsellers by category. It's broken down into top 100 paid and top 100 free. So if you wanted to see overall bestsellers:
1. Go to
www.amazon.com.
2. Choose Kindle - eBooks from the tree on the left.
3. On the right, choose the link that says Bestsellers - Kindle Store : Kindle eBooks - Updated hourly.
4. Browse the two columns, first being top 100 paid, second being top 100 free.
You can use similar methods to peruse top 100 paid and free for all the various categories.
-HM