Ideas, of course, are the seeds of any work of fiction or nonfiction. But until an idea is fully developed, until you can envision its beginning, middle and end, that one idea might not be enough. The experience of writing for pages about an idea and ultimately getting nowhere (or getting a pile of rejections) has taught many writers to outline their books before they begin. But if the thought of an outline sends shivers up your spine, at least thinking your idea through and making sure it merits months of writing can save you future frustration.
Ideas for Fiction
A lot of writers, especially when they're beginners, get ideas for fiction from their own lives. This can be useful for several reasons: you're emotionally invested in the topic, you can relate directly to the main character, and if the situation actually happened to you, you're less likely to be unconsciously basing the story on a book you've read. But remember, just because you find this thing that happened to you or your child fascinating, it doesn't mean it will be fascinating to thousands of potential readers. Very often, a real-life event is just that--an event. It's a vivid scene you recall with pleasure, or a family joke that's repeated over and over. It evokes strong emotions when you remember it, perhaps you even look back on an event as a turning point in your life. But only rarely does reality provide a plot.
When writers stick too closely to what really happened they fail to develop the elements necessary for a good story: a believable main character who is faced with a problem or conflict, mounting tension as that character tries to solve her problem and experiences setbacks, and a tension- filled climax followed by a resolution that's satisfying to the character and the reader. If your main character is really your son, you might not want to get him in trouble or throw rocks in his path. But you have to. It's the only way you'll create a story that will keep readers hooked and wondering how it will end.
Speaking of endings, if the resolution of your story comes too easily, it's probably obvious and predictable. Try mixing up real life and have the situation evolve in a different direction. Surprise yourself, and you'll surprise an editor.
However you get your idea, focus first on whether it's a plot or a theme. Many times, an initial idea is really the underlying meaning of the story, what the author wants to convey to the reader. Themes should be universal in their appeal-- such as friendship, appreciating one's own strengths, not judging others too quickly. Then play around with the sequence of events until you develop a plot (what actually happens in the book) that makes this theme clear to the reader. And remember; if you're using a childhood incident as the foundation of your story, tell it from your childhood viewpoint, not how it feels to you now as an adult.
Ideas for Nonfiction
Your nonfiction book should be based on something you're truly interested in and passionate about. After all, you'll be living with this idea for many months. The key to successful nonfiction is to take your idea and approach it in a way that no one else has ever done before. This means doing most of your research before you begin to write. Don't settle for the most easily-found information on your topic--your readers have probably read the same information. Keep digging until you find an aspect to your subject that strikes you as unique. Then search through the library and book stores to make sure no one else has already beat you to it.
For a nonfiction idea to become a book, you need enough information to fill the number of pages necessary, depending on the age group for which you plan to write. Younger children need a foundation of basic facts, but you can also get fairly detailed within the scope of the approach you've chosen as long as you explain concepts in a simple and straightforward manner (how animals hibernate, why insects are different colors). Older readers can draw on a broader foundation of knowledge, and infer connections between your topic and related subjects. A detailed outline of any nonfiction book is essential to help you see if your idea has enough substance and originality, or if you need further research before you begin writing.
Whether it's fiction or nonfiction, your idea should mean something to you, but also have the potential to mean a lot to your readers. Think it through, add to it, take the nonessential elements away, and make sure it has a beginning, middle and end. Only then will your "idea" turn into "an idea for a book."