You know the old adage, "Never judge a book by its cover." But all surveys on this topic point to the fact that book buyers do, in fact, do this. In a physical setting, the average buyer's first impression is the cover, followed quickly by the title and subtitle, then they typically turn the book over and read the back cover copy, and if they're still interested, they'll open the book and look at the table of contents.
The digital space is causing somewhat of a shift, but in a way that is making a book's title all the more important. The thumbnail size of your cover in most digital shopping spaces is too small for the artwork to significantly influence buying decisions. This moves your title and subtitle to the top of the list.
Typically, your book proposal is not going to include a cover for obvious reasons. But in my experience, acquisitions editors go through a pretty similar review process. This means your title and subtitle are paramount.
Your title and subtitle are the lenses your prospective agent or editor puts on and sees the rest of your proposal through.
One way to think of your title and subtitle is your book's promise and premise. You are communicating right up front what the main take away from the book will be for your reader, and how you will deliver on that promise. The same is true in your proposal. The remainder of your proposal will be evaluated based on how well you are delivering on the promise and premise in your title and subtitle.
Now, having said all of this, I'd recommend holding your title and subtitle lightly. They often change from proposal to publication. But do not let this knowledge excuse your work on this. You want to come up with the very best title and subtitle you can because of the impact it will have on the evaluation of the rest of your proposal.
The above applies to non-fiction. Fiction is a different animal. I'm not aware of any real hard and fast rules in fiction titling other than you want something compelling. You want to engage the emotion of the reader in some visceral way, and this is an art form. But when it comes to non-fiction, I also often get questions about more obscure titles. What about successful books like Blue Like Jazz or Velvet Elvis? All I can tell you is, sometimes they work, most of the time they don't. Unless you are an established author with a ready audience waiting for your next work, you need to broadcast clearly what your book is about, and your title and subtitle are where you do that.
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