The Secret of Cedar Tree Mansion is in the series titled Cedar Tree Mysteries.  This book follows Romance On A Deserted Island - Or Is It?  The main characters are Penny, Ron, Abby, Megan, Christy, and Randy.  All of the main characters bought a mansion together so they could live together.  The girls also have a book club, and they are amateur detectives.  Their book club meets in their small house that is in the back yard of their mansion.  They receive short stories and puzzles from an unknown source.  These stories help them to solve their main mystery.