What makes a subtitle great?  A great subtitle reveals a thesis and a promise. The thesis of the book is a statement that takes the conceptual format of “if ___, then ____.” The subtitle of Mel Robbins books The High 5 Habit is “Take control of your life with one simple habit.” It implies that the thesis is that one habit is adopted, control over life is achieved. We know exactly what this book is going to teach us. We might not know what the habit is yet, but it has a big promise: Control over life.

The Four Agreements is another bestselling book. Its subtitle is simply, “A Practical Guide to Personal Freedom” which tells the reader exactly what the book is about. It tells me the promise is freedom and that the thesis of the book is that by following a plan of alignment with some action (the ‘guide’), big things will happen. The back of the book tells me the promise will be “a new experience of freedom, true happiness, and love.”

A good subtitle tells the reader exactly what the book is about, and even how the book is organized. The subtitle might tell me if it a guide, a list, a skill, or a concept. It also shares the promises or outcome of reading the book for the reader.

I think a good subtitle captures some keywords. After all, most people are going to discover your book online, and by making sure a keyword or two is in the subtitle, you are increasing the likelihood of sales. In Martin Seligman’s book Authentic Happiness, he subtitles the book with the searched for keyword ‘Positive Psychology.’ The subtitle is also capturing the central idea of the thesis. “Using the new Positive Psychology to realize your potential for lasting fulfillment” is the full subtitle of his book.

The trend of subtitles in recent years is longer subtitles like Seligman’s, which has twelve words. However long yours is though, you should avoid redundancy: don’t say the same thing in two or three different ways or make a list simply because you want to cram keywords in it. A bad example subtitle for a book about overcoming depression might be: “A guide for depressed people to finish the right combination of medicine, personal change, and dietary changes to feel happy, fulfilled, and have meaning in life.” This example is wordy, redundant, and has an entire list of three things - happy, fulfilled, and meaning - that all mean essentially the same thing. A much better subtitle for this hypothetical book might be “Three Proven Strategies for Claiming Your Happiness.”

Amazon KDP will require that your title and subtitle together have no more than 200 characters. That will include spaces between the words, as well as the letters and any punctuation marks. You cannot use the subtitle to promote a website address, or to offer anything not included with the book (such as a free audio version or some other give away or advertisement.

I do like subtitles that speak to the reader. Using the words your or you become subtitles that literally make the reader see themselves in the book. Bob Martel did this in his excellent book The Magic of Aesop: How to Use the Wisdom of Aesop's Fables to Spark Transformational Change. I also like subtitles that answer a question. An example might be Person of Interest: Why Jesus Still Matters in a World that Rejects the Bible which is a current bestselling in Christian apologetics.

As you consider your subtitle remember that the purpose of the subtitle ultimately is to encourage the reader to look inside. Ask yourself, “Does this make me want to look inside?” If you are not convinced your subtitle is good, you can be sure your reader does not like it either.

 Just as your book title might change, so can your subtitle in the end. The subtitle can be used as a guiding star, however. If you subtitle your book “The Eight Most Effective Ways to Fight Off Bears,” you’re giving yourself a reminder to stick to the script and not veer off or scope creep into strategies for fighting off geese instead. When you are in the middle of writing your book and are unsure of what to write next, look at your subtitle, the thesis, and the promises you have made, and always write about that. It will help you stay focused and stay on track.

You might also realize that your book does not need a subtitle. Many best sellers have no subtitle. The subtitle is not a requirement. While all of my current books do have a subtitle, I considered not using one in my book The Seven Most Effective Methods of Self-Hypnosis. This title has the keywords I needed for search terms and tells the reader the format of the book and clearly what promise is made. The title stands on its own. Although I eventually did choose to use a subtitle, I probly would have done just as well without one.