It makes sense that authors are creating novels in which the main character(s) are “of a certain age,” and in Tess Gerritsen’s series “The Martini Club,” she has not only embraced that concept, she has flown with it. In “The Shadow Friends,” the third novel in the series, the usual crew are assembled for a conference on global catastrophes in their small town in Maine, when a catastrophe actually occurs. It’s not of a global nature, and it appears, at first glance, to be a simple case of food poisoning. But with this group, we know there’s more.