When the words don’t come, Richmond author Mary Burton turns on the oven. “It’s my delaying tactic,“ Burton said with a laugh about baking. “The running joke is, you can tell how the writing’s going by the number of cupcakes on the counter.“ Burton, who has written historical romances and these days focuses on romantic suspense novels, came up with a recipe for lemon-cherry teacake cookies while working on “Christmas Past,“ a novella for “Silver Bells,“ a collection of four short novels by Burton and three other authors. “Silver Bells” spent several weeks on The New York Times best-seller list.