I have used my Amazon account many times to purchase kindle books but my account has changed suddenly to purchasing the books for my i phone , which i do not want , how can iI change this back to my book again
Go to "Manage Your Content and Devices" in "Your Account". Are all your Kindle books listed? If they are, go to the Devices tab. Is your Kindle showing as registered there? If it is, you can make it the Default device. If your Kindle device isn't listed, you'll need to register it to that account.
When you buy a Kindle book it's added to your Amazon account and can be downloaded to any compatible device or app registered to that account. Not all books will be compatible with all devices.
If, in "Manage Your Content and Devices" you don't see all your books listed you have more than one Amazon account and will need to speak to Customer Service.