If I buy a book from Amazon on my iPad, can my wife also read it on her iPad?
Accordin to the discussion below, you can. There appears to be a limit of six simultaneous users per book.
We have ipads not Kendles so don't know if works same?
To read the books, you would be using the Kindle for iOS software, so it all still applies.
As Bruce says, you use the Kindle app to read the ebooks, and that's all Amazon care about.