[Cherise]

Where do click-throughs to my book page come from?
Percentage of shoppers who view the page and then go on to buy the book.
Percentage of book-page-viewers who interact with different portions of the page. (How many open the full "Editorial Reviews" section. How many want to see all the reviews? How many view or download the sample? Etc.)
General demographic data, if Amazon has it. (What percentage of buyers is 18-25? What percentage is female? Etc.)
On which book pages is my book showing up as an also-bought?

The first one of the above strikes me as by far the most important.

THIS!!!!!

This is even better:

Definitely where clicks to the book that result in a sale come from. If we know where readers who bought the book came from, we know what methods of marketing are working best.