The book is being sold for 99 cents on Amazon, because Amazon does not allow authors to set the price of a book as free. When Amazon learns the book is free elsewhere, the company will match the price. Amazon hasn’t matched the price to free yet. Hopefully, they’ll make a change soon. If not, feel free to alert them yourselves, as they take customer concerns seriously. There’s a link called, “tell us about a lower price” at the bottom of the Product Details section (I’ve circled it in a screenshot to the right).
If you haven’t read Dr. Carter & Mrs. Sinn, do check it out. It’s a fun, short frolic. Here’s the book trailer.