Who Needs an Agent?

You do, insists Editorial Ass.

She does say that her advice applies doubly to fiction writers -- which is absolutely true -- but I would just add, mildly, that this isn't as true for markets that aren't aimed at a broad general audience. If you're writing for a professional audience, of psychologists, accountants, engineers, software developers or what-have-you, agents can be useful in some cases but aren't as necessary.

If you've written a novel (or a non-fiction book aimed at a mass audience), though, she's 100% on the money: don't try to submit it to publishers without signing up an agent first. And if you can't get an agent for that project, it says something very strong about the marketability of your current work.