Research other books that are similar to yours and find out which agents or editors picked up these books. No matter how well crafted your proposal letter might be, it will be rendered useless in the wrong hands. It's vital that you find a relevant person, one who has already shown that he knows how to handle your genre and style of book, to whom to address your proposal. Referencing these other works in your proposal will serve to quickly paint a picture of the type of book you have written and quickly allow a decision to be made as to whether it is the "right" kind of book for your recipient. It also shows that you have taken the time and effort to locate the right person to receive your book.
Use a standard font such as Times New Roman or Garamond. Using an "interesting" and stylized font might seem like a good idea to make you stand out from all of the other writers out there, but it only serves to detract from the agent or editor's perception of you. Similarly, don't be fooled into thinking that writing in colored fonts will make your proposal any more likely to succeed. In fact, the opposite is likely true. Be sure to be consistent with your formatting throughout the letter. Don't be tempted to change fonts, for example, to quote a sample of your own writing. Choose one (standard) font and stick with it throughout.
Write a one-sentence introduction that states to the agent or editor exactly why you are contacting him and affirming that you have not done so haphazardly. This is where your research from Step 1 comes in handy. Open your proposal letter by stating that you are contacting the agent or editor because he also represented TITLE by AUTHOR and state that your book is similar.
Write a short description of your book --- no more than three sentences. Be sure to be as specific as possible in such a short space, perhaps explaining several of the themes covered by the poetry in the book, and the locations and time periods touched on. If possible, use one of your sentences to create a compelling comparison. Screenwriters Dan O'Bannon and Ronald Shusett pitched one of their scripts as " 'Jaw' in Space" (the movie was "Alien").
Write a brief biography about yourself. Try to keep it short, mention any worthy publication credits, any relevant industry contacts and be sure to mention whether you have a track record.