A few years ago, Anthony Horowitz brought out a superb Sherlock Holmes pastiche called “The House of Silk.” That phrase could easily serve as the design label for anything written by Horowitz himself. His recent books — especially the metafictional mysteries “The Word Is Murder” and “Magpie Murders” — are as elegant and richly patterned as a Hermès scarf. Certainly, anyone seeking a few evenings’ respite from the emotional roller-coaster of last week’s election need look no further than his latest, “Moonflower Murders.”