Copywriters should read poetry. Frequently.
Unlike most prose, poetry is less about telling a story and more about using language in unexpected and creative ways. Poetry avoids cliches. It evokes images. It leads you down a path you hadn't planned on traveling.
And isn't that exactly what you want to be doing as a copywriter?
I love Sandburg and Rilke and Basho. Shakespeare's a given. Two of my contemporary favorites are Robert Hass and Billy Collins. Go read a collection. It will make your pen fantifluous.