After doing this for a while, you should have a better idea what this book will be about. At that point, I recommend you make some kind of an outline. If this will be fiction, perhaps start with the 8 Elements of Plot. If the book is to be nonfiction, try to come up with a one paragraph summary of your topic and the core message of the book. Decide who will be your audience. You want to tell them something they will be glad to know. Then decide on the arguments, evidence, ideas, information, etc. you need to prove your thesis. Those may become chapters. Doing some research on your topic will give you more ideas.