I had a writing coaching question recently, from someone who has developed writer's block because she is afraid that she will inadvertantly reveal too much about her own past in the material she's writing. While it's rare for this fear to actually block a writer totally, it's just one example of feeling that one is not in control of one's material.
The solution I recommend is doing a mind map first. If you're not familiar with mind maps, have a look at the illustration below, which is a rough mind map of the central part of this post. If you want to learn more, look at any of the books about it by Tony Buzan, or search 'mind map' in Google. You'll understand the concept instantly: write the topic in an oval in the middle, and let lines radiate out from there, starting at the one-clock position and going clockwise. On each line you write a word or phrase that indicates the sub-topic. Each line, or branch, can have sub-branches coming out from it to carry additional details, and even those sub-branches can have more coming out of them.
You could start with one general map for an entire book, then a separate map for each chapter. Sometimes the first version just gives you a place to dump every idea you have about a particular topic. When you have that overview, you can organize it and do a new mind map that reflects that order.
If you have any questions about the process, let me know at [email protected]. Good luck and let me know if this strategy helps you.