I recently did just this, adding the number in the subtitle. So now, for example, Secrets to Die For has the subtitle "Detective Jackson Mystery #2." I did this as a convenience for readers, but I'm watching my sales to see if it has a negative effect. I worry that some readers who might be attracted to a single book or title might change their mind when they realize it's the third in a series. Which is why bookstores and publishers do not like to have the number on the book. It's a risk. As information: All my Jackson books can be read as standalones.
L.J.
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