Tasha, definitely check with publishers with a strong proposal or query first, outlining what you intend to cover subject-wise. Make sure that your credentials are in order, that will give you some credence and platform for writing about your subject. More than ever, nowadays, publishers are looking for academics, with solid professional backgrounds to script educational, social, or scientific texts. Of course general subjects can work well if they haven't been beat to death.
In 1988 I decided to write the first ever book on Garage Sales--did up the proposal and three chapters, sent it out, and it was accepted imediately by three houses. I took the one I liked and signed.
Second book had to deal with my profession--auto mechanics. So I decided to write a consumer warning book about automotive rip-offs. Again it was snatched right up.
Third book was about the third biggest ice age discovery of Pleistocene megafauna in my city ten years ago. Again I wrote up a complete proposal and 100 pages. Not remembering my past experience with non-fic books, this third installement has met with opposition because I am not a paleontologist, therefore I'm not considered a credible source to even document this historical discovery. I didn't have previous articles published about this find, which might have helped. Being a great science writer did not count in my favor in this case. Which equaled: weak platform, and non-narative.
I didn't have an agent for any of these--went straight to the publishers. Non-fiction accounts for just about 70% of all books sold, so it's a much easier route to persue. Just keep in mind, have good experience or a solid credentialed platform for an academic work. If you want to write a diet book, it sure helps to be a doctor like Atkins, or Stillman. Or even, a dietitcian (sp?)
I think that if I ever write another non-fiction book, it will be about subject matter that does not require a PHD. Cryptozoology and Ufology, come to mind, except that I'm damn willing to bet that the guy who has a degree in zoology or biology will get his book on Bigfoot published a hell of a lot sooner than I will. As for UFO's, an astronomer will probably push me out of the market place.
It's been said that if you have a zeal and the insufferable desire to write about your favorite subject, then you can submit it with the hopes of publication. I hate to be the bearer of bad news, but that is certainly not the case in this day and age anymore. For less topical subjects, I believe the amateur can arrive at a certain amount of sucess. But I would not, as a reader, like to learn how to fly a plane from a book that was written by a model plane builder, if you catch my implication.
Triceratops