If those books were bought on Kindle Store most of them will have DRM protection and you will not be able to just copy the same file from one Kindle device to another.
Not unless you strip the DRM before transferring the files to a new Kindle device.
Amazon's DRM locks the files to a particular device (based on device's serial number IIRC).
So the choices are either re-download those books (in batches of 10? books at a time) from your Amazon account or deDRM the existing copies (if those aren"t in KFX format

) taken from the old Kindle.
As always all book files are in "/documents" folder...