Hi there and welcome
It is possible these are sample books, but you can delete these
To do this, In the library you will see all your books are listed clearly. Press and hold on the book cover, a menu that offers several options will pop up. Tap "Archive", then these selected books will disappear from the library
If you choose Delete then they are gone for good, so this is why I advise to Archive just in case you would ever want them again
Let me know how you go please
you are in your Library looking at your list of books?