Although some non-fiction publishers prefer to work only with agented authors, you don’t have to have an agent to start with.
There are many things an agent can do for you as a beginning author. An agent is supposed to help you find a publisher and negotiate the contract, for which you will generally part with 15% of your earnings. Agents can provide a valuable entrance to the publishing business and some of the best will even give you help managing your career, but you shouldn’t sign up with an agent just because you think you need one. As you’ve seen, finding publishers can be easy and fun. For information on negotiating contracts, How to Be Your Own Literary Agent will tell you most of what you need to know. You might, therefore, be comfortable foregoing an agent entirely if you enjoy researching and contacting publishers and you are willing to consider negotiating your own contracts.
The moral of the story is: “Don’t get an agent until you know why you need one, then get the best you can find.” Stephen King says that if you’re a bad writer, your agent gets 15% of nothing; if you’re a good writer, agents will come looking for you and you can pick a good one.
Warning: Agents range anywhere from “great” to “terrible.” Moreover, an agent who works well with one person may be terrible for another because of personality and style differences. Ask your contacts who they like to work with and why. If you’ve already done a book or two, ask your acquisitions editor who they like to work with and why. Their preferences in an agent might be 180 degrees from your own goals, but it’ll be more information for making an informed decision.