Agents typically collect 15% of any income from your book, Jane Friedman writes. So how do you know if you need an agent? "If you want to be published by one of the major New York houses (e.g., Penguin, HarperCollins, Simon & Schuster), then you more or less need to have one — and want one on your side. If you’re writing for a niche market (e.g., vintage automobiles) or wrote an academic or literary work, then you might not need an agent." Plus tips on selecting agents.