They Never Die Quietly by DM Annechino
Sami Rizzo, a single mother, is assigned to the investigation of a psychopathic murderer who is crucifying women and taking their hearts as souvenirs. Even though he typically captures the women along with their children, he released the children when he's finished killing their mothers. Sami and her partner, Al, (romantic interest) don't have many concrete leads when Sami gets the bright (read: stupid and so cliche it's ridiculous) idea to use herself as bait by going on a date with a man who fits many of the characteristics described by other witnesses and by the children of the women who were abducted. Even admitting that what she is doing is dumb when she accepts a dinner invitation from Simon -- she goes ahead with her harebrained scheme and ends up locked in the basement of his isolated home.
As far as I'm concerned, the novel's entertainment value and believability ended on page 146 and limped to its predictable climax and ending a very long 130 pages of Sami in captivity later.