Susan  Fox
Susan Fox asked Robert Dugoni:

Mr. Dugoni, Having read many of your books I highly recommend any books written by you. Being an avid reader I’m a member of Amazon’s Kindle Unlimited Books & your books are often available on my subscription. I read about a book every 1-2 days. Your plots are ingenious & very captivating. My question is what makes you decide to turn a book into a series? Susan Fox Twitter @gagasgarden

Robert Dugoni It sort of happens. Sometimes it is the publisher, other times you feel you can't tell the whole story in one novel and indicate to the publisher what you would like to do. Some books are just single books, like The Extraordinary Life of Sam Hell. So it varies.
Thanks for asking

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