"Can I distinguish between good and bad?" -- 2 Samuel 19:35
Unless you like reading repeated references to a beheaded body, exposed male anatomy, the symptoms of indigestion, and performing sexual services for pay, you may find this book more disgusting than funny. The humorous sequences that aren't raunchy fall back on the usual gags (burning things up, cars being wiped out, and inappropriate suitors).
The main story lines of Finger Lickin' Fifteen involve Lula unexpectedly observing a murder and being hunted by the killers, Lula entering a cooking contest, Stephanie helping Ranger overcome a threat to his business, and Stephanie and Lula bringing in people who skipped their court dates. On the romantic front, not much happens. Stephanie is angry with Morelli, and she doesn't find Ranger all that hard to ignore sexually.
Unless you like raunchy humor, you may find that you'll be happier skipping this book and hoping that number sixteen will be more appealing.
This book just didn't fit into the series. It seemed more like a writing exercise to see how far an author could "push" humor into regions where it doesn't usually go.