What is the author's style in The Pigman by Paul Zindel?

An interesting feature of the book is the occasional insertion of handwritten elements, such as John and Lorraine's signatures on an "Oath" to tell the truth about the incidents described in the book; some graffiti John writes on a desk; and some pencil- and-paper games Mr. Pignati plays with them. The book also has a page from a booklet on funeral planning, a bill for a funeral, and a piece torn out from an advice column. These elements add realism and immediacy to the story, making it even more believable.