Can a Kindle owner use any Amazon Gift Certificates that I send them to pay for Kindle e-books? I can't see why not but it would be nice to have it confirmed.

Digging deep into the Amazon site I found that gift certificates can be used to purchase Kindle ebooks. However, they can not be used to pay for subscription content from the Kindle Store; I assume that they mean things like newspaper subscriptions.

You add the gift certificates to your Amazon account (it tells you how to do it on the certificate) which puts your account into credit, and you then buy whatever you like.