If there is only one book that is read by all three as common, since it also being said that the rest of the books are different, it would be further simplified in such a way that Sandra read 4 books, deacon read 5 books and breanna read 6 books. Coz have to exclude that common book from three of them. This will tells that the total (4+5+6) are of different books. So 8 different books might not be the right answer. Unless there is a further mention of books read in common by any two students, we cannot say though..