'Girl on the Train' sells 1 million copies

Publisher Riverhead is tooting its horn over the latest hot news for the runaway hit The Girl on the Train, and why not?

Paula Hawkins' debut psychological thriller passed the 1 million-copies-sold mark this week, with impressive speed. It's a phenomenal number for a novel by a previously unknown author.

The sales numbers are "very impressive," says Carol Fitzgerald, president of the online Book Report Network. "In a sluggish market during a tough winter it's a clear runaway winner."

Train, this year's Gone Girl, is No. 1 on USA TODAY's Best-Selling Books list for the third straight week. (The full list will publish on Thursday.) It was a hit from the get-go, landing at No. 2 on Jan. 22. Train first hit No. 1 on Feb. 12, then slid back to No. 2 for one week as Fifty Shades of Grey briefly reigned.

Despite early high hopes for Hawkins' book, the publisher started out with a modest first hardcover printing of 40,000 copies when it released Train on Jan. 13. (The digital version is outselling print this week, according to USA TODAY's data.)

Reese Witherspoon is the latest celeb to hop onboard Train. This week on Instagram she raved: "I don't know who you are #PaulaHawkins but you kept me up all night reading!"

(For the record, Hawkins is British and previously published novels under a pseudonym.)

Other stars who've reported on Instagram that they're on the Train express: Bachelorette Andi Dorfman, AJ Michalka of ABC's The Goldbergs and Jaimie Alexander from Agents of S.H.I.E.L.D.

The Brit-set novel is told by multiple female narrators, including the young alcoholic divorcee Rachel. She becomes involved in the case of a missing woman in her old neighborhood after seeing something suspicious from a train. Movie rights have been snapped up by DreamWorks.

Girl on the Train "has strong and solid word-of-mouth buzz and is billed as 'a quick read that you can't put down,' " says Fitzgerald. "It's appealing to dedicated book lovers as well as the casual one- to two-book-a-year reader. In the current entertainment market obsessed with binge watching, people are intrigued by a book they can speed through the same way — and talk to everyone about."