I just got a Kindle Paperwhite to replace my old Kindle. How can get my existing books onto the new one?

No you only deregister if you no longer have it. Just register the new one to start using it. I have a paperwhite and a fire tablet. There is an option to sync the books so you can use either one.

Kindle Paperwhite is associated with Amazon Account. You will need to create new Amazon account and follow the steps in this link to register your new Kindle Paperwhite with Amazon.

http://www.amazon.com/gp/help/customer/display.html?nodeId=201235720

Q: Can I transfer my existing Kindle content (including apps, E books, and movies) from my old Kindle device to Kindle Paperwhite or Kindle Fire HD?
A: Yes, all of your past Amazon digital purchases are automatically backed up in the Amazon Cloud so you can re-download them to your new Kindle or Kindle Fire anytime. When you receive your new device, you can download them from the cloud straight to the new device.
http://www.amazon.com/gp/help/customer/forums/kindleqna/ref=kindle_help_forum_gd?cdForum=Fx1GLDPZMNR1X53&cdThread=Tx1WEM06OFQ59MC

What you really need to do is add your new Kindle device to your
existing Amazon Kindle account. The good news is if you bought it from Amazon new, Amazon automatically registers it to your existing account. So . . .

Go to the Amazon website using your laptop or desktop computer and log onto your Amazon account.

1)On the upper right of the page you should see *Hello (your name)*
Drop down this menu by hovering your mouse pointer on it.

2) Select *Manage your content and devices*

3) If you're not logged in, enter your Username and password.

4) If you're browser is set to save passwords an log in you won't
need to do step 3

5) Select the *Your Devices* tab

6) All your Kindle devices should be listed there, including the new
Kindle Fire.

7) You can click on your old Kindle and *deregister* it.

8) You can make your new Kindle Fire your *default device* and
you can edit your other Kindle devices such as any Android
Device onto which you have installed the Kindle App.

9) The default device status ensures new books purchased will
automatically download to that device when it's connected to
the internet via Wi-Fi or 3G depending on what your Kindle Fire
is configured with.

10) You can click on the *Settings* tab to manage your credit card and other information.

You can easily load all of your books onto your new Kindle. Amazon retains a record of all your book purchases in an archive. When you register your new Kindle (and turn on wireless) it 'phones home' to Amazon and Amazon places an entry on your main menu entitled Archive. Just click on that entry and all of your past book purchases will appear and will automatically be downloaded to the Kindle when you click on them.

However, if your old Kindle contained any books, pictures, or documents that you did NOT purchase from Amazon, those items will not appear in the Amazon archive. You will have to download those items to your new Kindle the same way you installed them on the old Kindle. If the only location of those un-archived documents is the old Kindle, then this will become a two step process because you cannot copy from one Kindle directly to another. Instead you must copy the documents off the old Kindle onto a computer using the USB cable, and then copy the books from the computer onto the new Kindle by way of the USB cable or by email using an authorized email account. (To authorize an email account for sending books to your Kindle, please visit Amazon's "manage my devices" section. If the new Kindle is not registered, or if it is blacklisted, then you cannot use the email method to transfer the books. The only way to install books on an unregistered Kindle is directly via USB cable.)

Turn on your Kindle Paperwhite. On the home screen you will see under the icons on top --- Cloud ' Device.

Touch *Cloud*

This will take you to your library on the Kindle servers. Select those books you want and they will download to your device. Be sure you are connected to the free 3G or Wi-Fi so they can download to your device.

As they download you can click on *Device* to see them shown on your home screen.

I don't know of any way you can make a direct transfer from device to device. The key is all your books remain on theKindle Cloud so you have to download them from there.

Good Reading.

Contact Kindle directly and get a technician on line to walk you through the process.

As long as they are both registered to the same account. You can transfer them by going to 'Manage Your Kindle' or they will be archived, as long as you downloaded the books via Amazon.