This should be your first step. I said this before in an earlier post "Build your name before you publish a book" it's very important to know and understand everything when you're writing nonfiction. Becoming an expert can only add to your credibility. Even if you've worked in the field for a period of time you still should research your subject.
As suggested in "On Writing Well" you can never know to much. Visit your libraries and find out what they keep on the shelves, buy magazines, books and everything as related to your subject. Make sure you know your competition. See what they're doing and how you can contribute.
I agree with the suggestions made in this book. I also believe you should use the internet and see what's out there. Become familiar with people who may ultimately read your books and join forums, message groups and anything you can so that you are gaining possible readers or even information to help with your book.
Interview other experts, seek input and/or ask a family member. You should have someone you can ask how they feel about your idea. Become friends with your librarian, they have research tools and databases available. The more you know the better you'll be able to cover your subject.