“Go Set a Watchman,” the surprise second novel by Harper Lee, has proven to be a hit with readers. After being available for sale for just six days, it has sold 1.1 million copies.

Publisher Harper Collins announced the sales Monday, noting that the tally includes hardcovers, e-books and audiobooks. It’s the fastest-selling title in the publisher’s history.

As impressive as that is, it’s not 2015’s fastest-selling book. That honor goes to “Grey” by E.L. James, which in June sold the same 1.1 million copies in its first four days on sale.

One key difference between the authors is that James actively engages with her avid fan base through social media, while Lee, 89, has long avoided having a public persona. After winning the Pulitzer Prize for “To Kill a Mockingbird” in 1961, Lee’s last official media interview was in 1964. She declined to publish a second book for half a century.