Welcome, I am Cory.
If you have purchased the book in your Kindle (Amazon) account, you should be able to download it from there on to the Kindle once it is registered with the email address and password. If you want it on the Iphone, then you will have to download the Kindle App for Iphone and you will be able to read books on the Iphone.
Please let me know if this helped you.
Please let me know if you still need assistance with this.