Select a few agents and send each a letter which begins as appropriate:
I recently read ____, which is a wonderful book, and saw that you are ____’s agent.
Or:
My friend/acquaintance______, who is represented by you, suggested you as a possible agent for my book.
Or:
I read your article in______ about the book business, and was hoping that you might consider me as a future client.
Add the paragraph you’ve written about yourself, and then the paragraph about your novel.