Hi there and welcome
Exactly where on the Kindle did you see these?
and which model Kindle do you have?
1 minute, typing your response, please give me some time....
There would be two reasons for this.
Either the previous owner manually copied books over the the Kindle. This you can do as you do not only have to purchase them, you can also obtain the books from other sources and via your computer copy over the books
Or someone is purchasing books that you know (they would need your password) under your account.
Do you see any credit card transactions for these books?
yes what sorry? You see Credit Card transactions for it?