That delete / clear "removable storage" contents on deregister is the default behavior that can be (or at least it used to be able do that as recent as firmware 5.13.6 on PW3, that was the only time and the last time I needed to test that feature) disabled on Kindle with a jailbreak (and terminal emulator or SSH access) by creating an empty file with filename "DONT_DELETE_CONTENT_ON_DEREGISTRATION" at "/var/local/system/" location, so the full absolute path for the file is:
Code:
/var/local/system/DONT_DELETE_CONTENT_ON_DEREGISTRATION
IIRC deregister clears
everything from "/mnt/us" (= what you see as a removable drive when connecting via USB cable), not just books inside "documents" ( = /mnt/us/documents).