“As far as we can tell from our conversations with the book industry people, there has never been a trailer that did this,” said Paul Levitz, the president and publisher of DC Comics, which has printed 900,000 additional paperback copies of the novel since the trailer began running in mid-July. The book, about a conspiracy to discredit and murder a group of superheroes, was written by Alan Moore and illustrated by Dave Gibbons, and has been on the best-seller lists of Amazon.com, USA Today and The Washington Post. Mr. Levitz said “Watchmen” would have a print run of more than a million copies this year. Last year it sold about 100,000.