Kindle books on IPad
Assuming you have a recent version of the Kindle app, the simplest way is through the iTunes file sharing:  http://support.apple.com/kb/ht4094 has details on how to do it.

You can also mail books to your kindle address (if you choose the settings icon in the kindle app, you can see your "Send-to-Kindle Email Address"), and then it will sync to Amazon's cloud.

(edit)
Note that the book needs to be compatible with the Kindle app, and not DRM protected etc.