Agents make good partners, particularly if you don’t live in New York (but even if you do). If they’re competent they know the going rates and the best terms; they know which editors might be interested in your work. Don’t ask them for literary judgments about your projects. They’ll give them and you’ll be sorry you asked. (My agent friends will kill me for saying so, but literary acumen ain’t their forte. Nor should it be—that’s your job.) They’ll cost you at least ten, more usually fifteen percent of your writing income. You should see as you go along that their contribution of contacts and negotiating expertise adds at least that much to the total (and if it doesn’t add more, you ought to look elsewhere).