Keyword Data only Available for PPC
At this time Amazon does not make any kind of data available for organic customer searches, ie. there is no way to find out which keywords are generating how many clicks, views, sales etc.
Added Value for PPC
This fact makes the usuage of PPC even more valuable, as it is the only way to obtain actual performance data on your keywords, which can then help you prioritzing and optimizing them.
Relative Keyword Search Volume
Although Amazon also does not offer absolute search volume figures, Sonar's and Sellics' Keyword Research function will offer relative volumes.