once. To a thinking reader they will provide new food for thought every time he rereads it. Many people indulge in reading science fiction, fantasy or detective stories. Of course, there are some advantages and disadvantages of this kind of literature, often reffered to as «easy reading».
As for me, good science fiction and fantasy develop imagination, logical thinking, broader one's outlook.
The same could be said in favour of detective stories. They reveal such minute details of everyday life in this or that particular country that are of greatest interest for a curious reader. The masterpieces of this genre by Sir Arthur Conan Doyle, Agatha Christie and others, show the depths of psychological analysis, the insight into the human nature.