Double Indemnity by James M. Cain
Up until a few years ago, I was an insurance agent, but not like Walter Huff. Huff is good at his job, hard-working, and knows the insurance business inside-out. Then he meets a married woman who he falls for immediately. Her name is Phyllis and she has a thought, not even a plan, just a thought of what she would like to do about her husband. We don't realize at the time, but Phyllis knows exactly what she's doing, exactly what she's suggesting, and really has no qualms and is perfectly capable of holding up under pressure. She is not a nice girl, no matter how she seems at first.
It doesn't take Huff long to come up with full-blown plan. They'll kill the husband and they'll get away with the insurance money. He has been in the insurance game for a long time and he knows about every angle ever thought up by anyone to try and pull one over...