This is an excerpt from my book Self Publishing Made Easy: The Food Bloggers Guide to Writing, Publishing and Marketing a Cookbook, my comprehensive guide to creating cookbooks.
Choosing the right subject for your cookbook is probably the most important decision you will make. The best cookbook in the world will not sell a single copy if it is on a subject no one cares about. Regardless of the goals of your book, there are several factors that make a good subject.
If you have a successful blog, choosing a subject that relates to your blog is a great place to start. You are assured of an initial group of readers just based on a percent of your current fans purchasing a copy. You may also be able to reuse or rewrite large amounts of content you have already created for your website.
Leaning on your users for helping determine the subject matter is another great idea. You can reach out to your readers early on in the process and ask them what their favorite recipes of yours are, what they wish you would cover, and what they'd like to see more of from you. The answers to those questions can give you a real good idea of the direction you could go in for a successful cookbook. On a side note, they are also good questions to ask when trying to figure out how to grow your blog in general.
You also want a subject that you find interesting. If you are going to write 40,000 words on a subject it better be something you enjoy writing about! As your motivation comes and goes, enjoying your subject is a critical part to finishing your book and doing a good job with it. In addition, through recipe testing you are going to be eating a lot of the food you're writing about. So if you don't like fish, don't write about fish even if you found the perfect cookbook subject in that niche.
In general, you want a subject that is popular, but not so popular it is saturated with books already. A subject with no interest will never attract any readers, but a subject being addressed with a lot of cookbooks already means your book will have a harder time standing out.
A great way to find subjects that fit into this mold is to find emerging trends that are just getting started. You can get in on the forefront of the trend and capitalize on the emerging popularity.
If a subject is too popular, you may be able to find a smaller niche within that subject that isn't as competitive. For instance, instead of "Italian food" you could write about "Tuscan food". Or instead of "paleo food" you could focus on "paleo desserts" or "gourmet paleo food" to narrow your subject and avoid competition.
Conversely, if you initially wanted to write about "Tuscan paleo desserts" you might discover that there isn't a large enough market for that subject. You can then broaden your subject until the competition level becomes too high.
Some of my best selling cookbooks are on smaller, more focused niches. There might be less people interested in the book, but a much larger percent of them buy it because my book is the authority on the subject instead of just another cookbook competing for readers.