Sad Cypress Part I

I am currently reading an Agatha Christie book called Sad Cypress.  It is the first book that I have read written by Agatha Christie.  It is a tale about murder and a clever detective Hurcule Poirot.  A young girl has been poisoned out of jealously, and there is only one person who could have done it.  All the evidence points to a woman named Elinor Carlisle.  She broke off her engagement with her fiance because he was in love with Mary, the girl who was poisoned.  As this is my first Agatha Christie book, I don't know where this is headed.  I have a few ideas, but I hope they are all wrong.  I love when books take me by surprise.  Agatha Christie is supposed to be such a brilliant author because she understands the way people tick.  This book is told from various points of view with one chapter being completely written by letters (as in the ones that a person mails at the post office).  To tell the truth the only reason I decided to try an Agatha Christie book is because of a Dr. Who episode that was done about her.  I have not read very much British literature.  I am not very attached to the main characters; however, I am interested in how the story will end.  I'm over half way through and should have it finished by next week.