If you're in an anglo-speaking country, maybe consider something like Farnsworth's Classical English Rhetoric for common rhetorical devices in speechwriting (it also comes with numerous famous examples in history).
Not every piece of communication written has to be on the level of the Gettysburg Address or Pericles' funeral oration, but being able to transfer the patterns of great speeches over is a useful skill.
He's also written a book classical rethoric and another on metaphors.