[stumped]

are you sure you did not change your amazon account, sometime after buying the old "lost" books
if not, they should show under my content on the web site
to get rid of a book from cloud you have to explicitly delete from there , one book at a time, and confirm you really want to delete. there is no way it should happen by accident. I know that with apps you can archive them , so that they are still there but do not show up as cloud content on fire tablets until uou unarchive. not sure if you can archive books like that, but could be worth checking


i suggest talk to amazon.

purchase records shoudl still be on the web site - forever, in my purchases , even if u deleted email receipts.
you have to try really hard to lose all purchase evidence with amazon, unlike say ebay who only keep stuff for a couple of years at best.
and if amazon like you as a customer, they will probably give goodwill replacements or credits for a submitted list of lost kindle titles. amazon can probably tell you if you ever had a different amazon account, also.