This recommendation comes from Amanda. You can follow her on Tumblr and on twitter @ShoutTheBoogie.
Double Indemnity (1943) by James M. Cain
This brief but brilliant crime novel explores why ordinary people are driven to do evil. Here, Cain tells the story of an insurance salesman who teams up with the unhappy wife of a wealthy client to kill that client and collect the "accident" insurance (which pays double indemnity, of course). Double Indemnity is a classic of the genre.