Love this, Charlie! Waiting for you to build a "DN for poets" ;)

Running against most of the advice in this thread (egad!) my advice would to be to avoid most of the advertising books and study some of the master prose stylists like Joyce, McCarthy, Toole, etc. Granted, they won't teach you formulaic ways to come up with taglines, product copy, etc. However, they'll inspire the sort of whimsy and wordplay that delights your users/readers, and will help set you apart from all the other copywriters doing things "by the book".