There is 3 different kids of keywords being talked about here...
1) those in the meta data of the ebook - these are probably useless
2) Those for SEO - these are religiously argued till the end of time. I would not spend much time on them, but adding some well thought ones is prudent. After all if you can get 6% of the search traffic to your site, it would probably crumble under the load.
3) Those in Amazon, and other stores - THESE are important. These get right. They do impact sales, IME. These are the ones I was talking about. Most people will not find your web site in Google, the will be looking in Amazon/etc for books and find you there. So focus your efforts there.