A literary agent sells your book for you. More specifically, they know all the editors at all the publishing houses, big and small, who want to publish a book like yours. They maintain those relationships so that when the perfect book (yours!) comes up, they are a trusted source.
Agents are also up to date on the direction of the market, and they'll advise you on what to write next, or how to best revise your work to receive the right audience.
They'll also negotiate your deal, usually getting you a lot more money than you could have gotten on your own. An agent is your advocate, the person who is on your side. Most of the big publishing houses won't take your submission without an agent, and even smaller houses prefer agented manuscripts.
Now. How do you go about picking one?
Start with the big resources:
Compile a list of agents who are active, accepting submissions, and represent your category and genre. This list will be very long. Hundreds of names. Now you'll narrow them down based on your own personal criteria. Some of the more common criteria:
- Have they made a sale to a publishing house you know and respect in the last year?
- Are their current clients experiencing success?
- Do they respond in a reasonable amount of time? Even after you've been signed?
- Are they reputable?
Those answers can be found on sites like Preditors and Editors as well as the Absolute Write Water Cooler forums. Just google "absolute write <agent name>" and you'll be taken right to the page where everybody else is talking about this agent.
Once you narrow the list down once, you can go a little further based on your personal preferences. Some things to consider:
- How tech-savvy are they? Do they expect you to be the same?
- How communicative are they?
- Her personable are they?
- Do their reading tastes mirror your own?
To find this info, you'll need to dig deeper. Twitter, blogs, personal websites (most agents will maintain a blog or tumblr in addition to their agency's official website). Find interviews they've given. Most agents will get featured on Writer's Digest or even on smaller blogs and websites.
By this point, depending on how picky you are, you'll have a list that's somewhere between ten and a hundred names long, and it's time to start querying :)