Great Start and Middle, Weak Ending

Years ago, The Pigman was a required reading book at my high school.The story starts out pretty interesting, but even at the middle its, hard to predict where the story's going. The end was completely unexpected. It didn't really go with the theme of the book. Except the ending, it's a pretty interesting and unique book. Who would've thought to write a book about the relationship between two teenage friends and an unrelated elderly man.