Also, you can't e-mail a Kindle book to another iPad. Each book from Amazon is device-specific and can't be shared like that (as a result of Amazon's Digital Rights Management, or DRM).
What you can do to share a Kindle book is to authorize that other iPad to access your Amazon account. That way, the person with the other iPad can just download the book from Amazon right to that other iPad.
All that has to be done via your account at Amazon, BTW.
Good luck.
Marilyn