This excerpt is from “Success, Your Path to a Successful Book,” by Maralyn D. Hill and Brenda C. Hill.
The best advice that we can give regarding agents is to research and read articles about them and by them on the internet. Then make the decision if an agent will benefit your specialty.
You may want to read The Renegade Writer’s Query Letters that Rock, The Freelance Writer’s Guide to Selling More Work Faster, by Linda Formichelli and Diana Burrell. It provides query letters that have worked, along with comments from the writer and editor.
If you want a traditional publisher, you will most likely need an agent. Very few large publishers will accept work unless it comes from an agent.