From David Starkey’s excellent Creative Writing,

Write a poem in the form of a letter. Richard Hugo’s book 31 Letters and 13 Dreams is a great source for these types of poems. Hugo addresses poems to close old friends and to newer fiends he doesn’t yet know well. He brings these people detailed news of his own life and asks for information about their world. However, because the letter is in the form of a poem, Hugo condenses what he has to say and present the material as eloquently—and as imagistically—as possible.