3 stars because I already kinda guess who the killer is . 
 (view spoiler)[ I first suspected Linda for her odd behavior, but then again Agatha has almost never chosen a teenager or a child as a murder suspect. Guess she doesn't want her novel to be messed up. So I first played suspicion on the husband/Rosamund. But again, every clue seem to be pointing at them, so that does not sound right. 
 Having read Murder at the Vicarage, Death on the Nile and Mysterious Affair at Styles, I remember the old trope of UNLIKELY pairing. We know that the least suspicious person is the lover Patrick, who found the body, and his weak wife, Christine Redfern. However, I felt suspicious when the book said that Patrick was left alone with the body. I smelled something fishy here - he could have messed up something. As the story progressed, I kinda lose interest in them, however, because clues kept pointing at Linda and Rosamund. However, right at the end when Poirot about to make his talk, I felt very suspicious towards Patrick and Christine. The old case also kinda make me feel it to be likely. So of course, my instinct point me towards the right murderer! (hide spoiler)]