Titles are important. Any author can agree to that. But are subtitles as important, less so, or possibly more important?
One consideration is how subtitles impact algorithms that help readers find a book. Subtitles that seem unwieldy because they are so long can actually boost sales on websites.
A second consideration has been around for a while: a descriptive subtitle tells readers exactly what they’ll get from the book.
Finally, a subtitle can indicate the author’s voice (funny, academic, etc.) or tone (the emotional quality of the work). That can be attractive to readers and thus generate sales.
Subtitles are clearly not more important than titles…but they are equally important.