Hello. My name is ***** ***** I can help you with your problem.
Connecting your kindle to your computer is not the way to download your books.
You registered your kindle with your Amazon account, the same email id and password ***** you use to login online to www.amazon.com.
Since you can see the serial number of your kindle in the Devices tab in the Digital Content section of your amazon account online, you can send books that you have already purchased with your online amazon account to your kindle by checking the Select box next to any book in the list in the Content tab of the Digital Content section your amazon account online and selecting Deliver in the Actions box.
Hello again, and thank you for letting me know about your login problems. I have an Amazon account and a Kindle app on my mobile phone. I do not have a Kindle, but my mobile phone is a proxy for downloading books from my Amazon account, from which I should be able to understand the process well enough to advise you.
My Kindle app shows up under the device tab on www.amazon.xom as a name, not a serial number. The name can be edited and changed to any descriptive name that I choose. This leads me to believe that there is nothing significant about either serial number which was assigned to your device when you registered it and reregistered. It is not really a serial number; it is just a unique name assigned by the amazon website software. It is not a problem that the "serial number" was different each time you registered your device. You can change it to a more descriptive name, such as "Your Kindle".
Regarding multiple users on your Amazon account, it is my understanding that there is only on unique login id on the account, which is the valid email address of the original user on the account. The amazon password ***** or may not be the same as the email password. There may be multiple Kindle devices registered on the account, but all users would use the same id and password, and all purchased content would be billed to the original responsible user.
Now, you say that it will not accept your login information. Apparently, you are able to login to the Amazon account on www.amazon.com, because you have described the Manage Your Content and Devices webpage witrh device tab and content tab. You are also able to lognin to your Kindle, because you need to be able to do that to reregister it. Therefore, I do not understand what your problem is.
I just noticed that my Amazon TV FireStick shows as a registered device on my Amazon account, and it shows a serial number, so I was wrong about that. If your serial number is ***** that can be corrected by performing a factory reset on your Kindle device.
If your Kindle has not been configured with the proper id and password, you will need to do a factory reset and start over, using the same id and password ***** the Amazon account. You can restore all your purchased content afterward, because it is yours and it remains in your account on the Amazon website.
If you are satisfied with my information and advice, please be sure to give me a Good or Excellent rating. If you are still having problems, please let me know. If you have any more questions, of if there is anything else I can do for you, please do not hesitate to ask. My name is ***** ***** I am grateful for the opportunity to be of service.
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How are you doing with your Kindle? Are you still having problems with registering it to your Amazon account?