Robert B Parker. He created Spenser, the Boston PI (brought to life by Robert Urich in Spenser for Hire). He also authored the Jesse Stone series (Tom Selleck did that one.) Over the course of his life he wrote around 70 books. Standouts include The Godwulf Manuscript, Early Autumn, and Walking Shadow, Night Passage. Like Elmore Leonard, this guy's known for his turn of a phrase. Really good writer. If you like PI and crime fiction, check him out: RobertBParker.net
I too noticed this pattern of everything getting increasingly darker. Also, if you listen to Hedwig's Theme in the first movie, it is very light-hearted. But by the end, the music has changed into a haunting and ominous tune.