David Sheff and his son, Nick, read at a Starbucks in Somerville’s Davis Square last week as part of their tour to promote Beautiful Boy, the memoir of the Sheff family’s struggle through Nick’s meth addiction, recently tapped by the coffee chain for its national bookselling program. (Right around the time this reading was taking place, as it happens, I ran into someone from Houghton Mifflin, and though I couldn’t get exact numbers, it was allowed that “everybody was happy” with initial reports of the book’s sales.) Their tour continues for the next two weeks, working its way to the west coast and then taking a northerly turn until they wind up in Seattle on March 17.