[header] How to decide on chapters for your book [title] Make a list of the main ideas you want to cover once you've chosen your topic and done some research. [step] Use what you've researched, especially the areas that interest you the most. Make sure you cover the most important aspects of your topic, as well, aspects that anyone in the field would expect you to cover. 
[substeps] The same goes for something like a biography. For instance, it would be strange to write a biography of jfk without talking about his assassination.