Most of the publicity agents that I work with are employed by publishers. The one that isn't is: Break Through Promotions (David Nunn) is who I get material from. I believe they work with the author's who aren't getting the publicity from their publishers and who want to get their book out to reviewers.
I know that movie stars have people who just make sure their names are in the papers all the time. But that's not something that I know anything about.
Most of the time, I believe that they (agents/publishers) want a built in pool of buyers before the publisher will take on a book especially in what they perceive as a niche market. Give the agent a write up of the intended audience for the book along with the proposal. Sometimes that helps.
Also, it helps to talk to publishers and publicity people at various conferences and such. We've (for the zines) have been going to the ALA Conference when it's in DC. Here's some dates and places (it was in Anaheim this year in July):
- July 9 to 15, 2009 - Chicago
- June 24 to 30, 2010 - Orlando
- June 23-29, 2011 - New Orleans, LA
- June 21-27, 2012 - Anaheim, CA
- June 20-26, 2013 - Washington, D.C.
- June 26-July 2, 2014 - Las Vegas, NV
For more information, visit their site.
These are huge. It's usually only $25 to get to the exhibit floor -- filled with publishers and publicist and here in DC filled the convention center floor about 3-4 football field of displays and publishers.
Talk to publishers at professional conferences. I know the American Meterological Association always has publishers at their events (usually no editorial staff but they do have the sales and publicity people, and often they're willing to talk about markets and stuff -- look interested and listen and they babble and you can pick up some good info on what their markets are like).
If the book is for lay-readers as well as professionals in the field, I'd not worry so much about an agent as finding a publisher you think would be a match with the book and checking to see if they take un-agented material. If they do give it a try. If not, then if you can meet an editor and give an elevator pitch at a convention or someplace similar, they might be willing to take a look without an agent then if they offer a contract -- finding an agent is much easier.
It's fairly difficult to prove you're famous if you're not infamous, but I have the feeling that the agent will be more interested in news articles and lay magazine publications and coverage.