The most common question about book publishing is this: Do I need an agent?
The answer depends on what your goal is. If you just want to get your book out there, the answer is no, you don’t need an agent. Even if you don’t plan to self-publish, many small and independent publishers out there accept submissions directly from authors. Some of them have periodic open submission calls, others just take whatever comes their way when it comes.
If you’re aiming somewhat higher, like, say, at one of the Big 5 publishers, or a movie deal, the answer is yes, you probably should find an agent to help you out. Big publishers are big for a reason: they have a gazillion authors on their roster already, and massive negotiating power on their (or their own) behalf, which makes them immensely attractive as a business partner. The problem is, that means they get flooded with submissions on a regular basis and most of those end up in the trash.
That is why agencies arose. Literary agents are like the first hurdle, the sieve that separates potential bestsellers from not-so-awesome junk. They go through the hard work of choosing something they believe has merit and then present it to publishers for consideration. Many, many, many publishers these days will only look at books sent by agents, or ones they specifically requested. Agents have clout, experience, hopefully a good eye for talent, access to people in the industry, and the business know-how of how to present a manuscript to them, negotiate on an author’s behalf and close the deal.
For that, they get a piece of the action. The first little bite out of your potential future income. The theory is that said potential future income can potentially be much higher with the help of an agent than without it. It can happen. It’s also possible that it won’t.
NOTE ABOUT UPFRONT FEES:
Different agents and agencies will specialize in different things, whether it’s specific genres, specific territories, or specific parts of the industry (such as bridging the gap between novel writers and the movie industry). One thing the reputable ones all have in common is that they do not charge upfront fees. Paying someone upfront means they already made money off of you and they no longer have any need to help you make money for yourself. It opens you up to a process much longer than you originally expected, and the risk of something called escalating commitment where the agent (who already got paid) comes back to ask for more money, and you pay them, again and again, figuring you already invested so much into this, you might as well see it through. Don’t. If any agent asks for money upfront, stay away.