The three phases of writing a nonfiction book are preparation, research, and writing. After you have finished preparing for your book project, it is time to sink your teeth into the research phase.
Online research is often the first step to researching material and ideas for a nonfiction book. You will begin by searching for articles on your subject in search engines. You will read through the articles you pull up, take notes, and maybe come up with a few chapters or subsections to add to your outline.
You will also want to conduct a search on Google Books (http://books.google.com/) and you will want to check out the The Online Books Page (http://onlinebooks.library.upenn.edu). Both of these websites can help add a wealth of information to your book.
Public libraries are a great resource when searching for mainstream topics. You may also want to check out any local college or university libraries. I have spent countless hours doing research at the Kutztown University library and I don't know what I would do without this wonderful resource.
Used Book Stores
Used book stores tend to have a lot of out of date books, but there is almost always a gem or two in the stacks. I usually go to the local Goodwill store that sells books for 25 cents a piece and buy a stack of books for current research and future research.
Talk to People
Depending on the subject of your book, you can also contact people in the industry and give interviews. You can include the interviews and what you learned from individuals in your book.
Do It Yourself
Finally, one way to gain a wealth of information about something is to go out and do it yourself. For example, if you are writing a book about a type of business, the quickest way to gain insider knowledge about that business is to work for one or set one up. Use yourself as a guinea pig to gain a new perspective and knowledge for your book.