Book is worth reading (SPOILER)
It may be hard to find; I found it at our local college library. The movie follows it pretty well but in the book, the character of Ellen is even nastier and more vindictive. After Ellen "accidentally" lets Danny drown, (in the book) she actually cleans and washes his body after it is brought back to the house. All the while she's thinking how she's going to get out of the mess she's in because she realizes her husband knows Danny's drowning wasn't an accident. Ellen's main fear is that her husband will turn her in to the police. So she decides to tell him she's pregnant, knowing he will protect her. In the book, she's thinking to herself that although she's not pregnant, she must make it happen as soon as possible.
The most bone chilling part of both the movie and the book is when Ellen decides to commit suicide by swallowing arsenic and staging it so that her sister and husband will be blamed. But her death in the movie was far more sanitized than in the book. Arsenic poisoning makes a person deathly nauseous and causes terrible pain, both of which were detailed in the book. In the movie, she died quietly, still beautiful, in full makeup!