Are these only Amazon books? If yes, get on WiFi and go into archived items/cloud on your old Kindle and download them from there. Or go to your Amazon account and send them to your old Kindle. You can also set your default device if you purchase from a web browser. Anything private/ not Amazon can be copied from/to documents folder via USB. I think you cannot do this with Amazon books, they are DRMed, most likely based on the device serial number and not your account. Thus going through official channels.