There is actually no need to "transfer" anything, so long as the kindles are all registered to the same amazon account.
If so, they will sync up automatically.
You may need to go into the Books folder from the kindle home page, then tap on the tap at the top for CLOUD, rather than Device.
This will show all your prior purchases, and as you touch on each one, it will download into the kindle and be stored in the kindle "device" so it is always accessible offline.
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