Alison Waines didn’t expect her two-year-old psychological thriller to hit it big.

But it suddenly took off this year, selling tens of thousands of copies, climbing to the top of Amazon.com Inc.’s U.K. and Australian charts for e-book downloads, and becoming a topic of discussion in book clubs.

“I’m making more money than I ever have before,” said the former psychotherapist.

The plot twist? Many of Ms. Waines’s readers bought the book, “Girl on a Train,” by mistake. They meant to buy “The Girl on the Train,” the best seller by Paula Hawkins that is being made into a movie starring Emily Blunt and Justin Theroux.

Of roughly 300 reviews on Amazon of Ms. Waines’s 2013 book since Ms. Hawkins published her novel in January, around 40 reference buying the wrong book. Many appear to have read the entire 429-page novel without realizing their error.