What is the purpose of book in story Harry Potter and Goblet Fire?

The book you are referring to in the story "Harry Potter and the Goblet of Fire" is "Mad Eye Moody's Guide to Defensive Magic". This book appears during Harry's fourth year at Hogwarts. Although it wasn't a primary component to the main plot, a character named "Barty Crouch JR" disguised as Alastor "Mad-Eye" Moody, uses the guise of being the Defense Against Dark Arts professor. One can assume the fictional book was authored by Barty so students can believe they were getting accurate defensive magical education through Moody, however Barty's intentions were actually sinister.