Wikipedia calls shtick a comic theme or gimmick.
About a quarter through the cozy, A Good Man Gone by A.W. Hartoin, I must confess to enjoying the comical aspects of depicting the main character as a Marilyn Monroe double, who puts that likeness to good use while trying to solve a mystery. The Kindle edition, by the way, is still free today (Monday) at http://www.amazon.com/Good-Gone-Mercy-Watts-Mysteries-ebook/dp/B00D1CPG5I
(I'm not sure how long the freebie will last.)
Other gimmicks can be found in Joanna Slan's cozy series featuring Kiki Lowenstein, a scrapbooker, as well as Lois Winston's cozy series with Anastasia Pollack, a magazine crafts editor.
Outside the mystery genre are Celia Yeary, Cheryl Pierson, Caroline Clemmons, and other authors at the Sweethearts of the West blog. Their shtick is using the West as a setting for their romances.
Can you think of other examples from your own novels or someone else's?
Excerpts at http://morgansbookslinks.blogspot.com
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