The question really isn't "should I get an agent." Instead, the questions should be, do I need an agent; am I ready to have an agent, and finally, how do I find the right agent?
Finding an agent can actually be harder than finding a publisher for your nonfiction book. There are over 800 agents listed in the 2009 Guide to Literary Agents, so it would seem like it shouldn't be that difficult to get an agent. A very important rule in finding an agent is that no agent is better than the wrong agent.
Writers submit their work to an agent by means of a nonfiction book proposal. The proposal is identical, except for the cover letter, that a writer sends to a publishing house. The cover letter, of course, is addressed to the agent and not to a publisher. When you are determining which agent to send your proposal to, check to see what books the agent has represented, which houses they are published with, and in what genres they are published. You'll want to choose an agent who represents work that is similar, but not competing, to what you are writing.
Agents will not edit a writer's work, but they will advise the writer about the quality of the work and the marketability of the work. The writing may be the most outstanding the agent has ever seen; the subject may be fascinating, but the agent knows (or believes) there is no market for the book. If there is no market, the book won't sell, no matter how well it's written. However, agents have been wrong and publishers have been wrong. That's why writers should always keep looking when their manuscripts are turned down by either agent or publisher. The writing may simply not be a good match. If you've done your homework in researching agents, you'll have a good idea before you send your work whether your writing is a good match for that agent. Agents and publishers can tell if you are blanketing the field with your manuscript, so be careful that you don't do this. Target each book proposal to specific agents and publishers.
New agents or "junior" agents who work under an established agent are usually easier to get than long-established agents. New agents are often working for smaller publishing houses which makes it easier to be accepted as a first-time author or an author who hasn't written too extensively.
One of the first questions that an agent will ask a prospective author is "how big is your platform?" The publishing industry is constantly evolving and platform has become paramount. It helps the agent know that you are doing work to help market yourself as an author, long before you try to get a book published. The only writers who don't have to be concerned about devising their own marketing plan are the few top sellers in the book industry.
If you have to already be marketing, developing a platform, and researching the publishing houses, then what's the point of an agent? If you feel you can not possibly "shop" your manuscript around with different publishing houses or that you can not begin to understand a book contract, then you will have to have an agent. An agent has network contacts in various publishing houses to know which places would best be suited to accept your manuscript.
The standard agent fee is 15% of any book advance and 15% of royalties. So, the agent will work to make sure you, the writer, have the best financial package possible. Plus, if the writer doesn't make any money, the agent doesn't make any money, so the agent has the writer's best interests at stake. With this symbiotic relationship.
it works best, if the personalities somewhat meld. An agent doesn't accept an author based solely on the one book being submitted. They want to know the writer is not just a one-book author.
Think of having an agent as someone who can help guide your writing career. With this in mind, you'll want to choose your agent carefully, if you are in the market for an agent.