"Nelson De Mille has taken you to The Gold Coast, he's taken you to The Charm School, and now he takes you to the edge of your seat." So begin the TV commercials for Plum Island, which was published as a Warner hardcover on May 23; its first printing of 500,000 was matched by a $500,000 advertising and promotion budget. Returning close to Gold Coast turf, the author sets his latest suspense opus on the North Fork of Long Island, N.Y., just off which lies the eponymous (and very real) isle. DeMille, a longtime Long Island resident, notes that Plum Island itself has long been shrouded in secrecy; in fact, he says, "The government admits that biological warfare experiments were done there in the 1950s." There's no secrecy about Warner's publicity plans, which include an eight-city tour that kicks off today, along with a 25-city radio satellite tour. Not surprisingly, "a ton" of signings are scheduled for Long Island bookstores, and movie ads will run on a number of New York screens.