Passion for their subject. You need to define and choose the kind of book you will write. It could be a general cookbook that covers all courses at a meal, listing appetizers, soups, salads, breads, meats, vegetables, fruits and desserts. A more limited cookbook could be devoted to one course or one ingredient, and might be called Breakfast Bonanza, Surprising Snacks, Onions and More Onions, or The Peanut Butter Cook. Regional cookbooks emphasize the food cooked and eaten by a geographically limited group, and could be called California Fresh, The Heartland Cook, or Kansas Memories and Receipts.
Ethnic cookbooks are limited to the cuisine of one country or people.
To avoid overdone subjects, limit your parameters, as in Fast French Cookery, Italian Vegetable Pastas, or Jewish Celebrations. Appliance cookbooks are centered around the use of one appliance, with ideas like How to Really Cook with a Microwave, You and Your Pressure Cooker, or The Basic Bread Machine Cookbook. These ideas will trigger your own. Select your topic and get started. First, formulate an outline which can evolve into a table of contents for the finished book.