Also when I mistaken downloaded a book in Janese called iBooks Author Iseem to have lost some PDF's that I had
previously downloaded.
How can I recover them ?
Well, just tap the 'Edit' button in the right upper corner; then press the book(s) you want to delete - a checkmark will be placed on the book - hit the red 'Delete' button to the left; a warning box will appear - hit 'Delete' or 'Cancel' - let us know if this works for you?
Concerning deleted books, the answer is maybe, i.e. did you purchase the books from the iTunes store or are the books in a back up on your computer?