on Jane Friedman site:

Let’s say you’re at a cocktail party and someone says I read this great book called The Hunted and it’s about this amateur detective who must evade a killer to solve a series of murders. Your reader is now your salesperson and sure, they know the book because they read it, but they likely read it because something about the book grabbed their attention. It probably was a combination of the cover, title, and subtitle. All of these contribute to a sale and if one of them is off, the sale most likely won’t happen.

Often overlooked in this equation is the subtitle. A good subtitle gives a book context and helps to pique a reader’s interest, but they can be tricky. Most authors don’t like to give away too much of their book, which is why you’ll see a lot of book titles that feel nebulous. Authors often feel like if they give away too much, the reader won’t want to buy it. Nothing could be further from the truth. You must whet the reader’s appetite with a benefit-driven title and subtitle—and this applies to fiction as well as nonfiction.

First, here’s what Amazon does not allow in subtitles: