As expected, the To Kill A Mockingbird sequel Go Set a Watchman is turning into a huge hit.

A spokesperson for Amazon says that Go Set a Watchman — which hits bookshelves tomorrow — is the most pre-ordered book on their site since the final Harry Potter installment Deathly Hallows which was released in 2007. Those who’ve been browsing Amazon’s Books section in recent weeks shouldn’t be too surprised, as the new novel has been at the top of the bestseller lists for a while.

And like Harry Potter, Go Set a Watchman is being kept under lock and key by retailers. “We’ve got our best security operations on this book,” said Mary Amicucci, vice president of adult trade and children’s books for Barnes & Noble to the Wall Street Journal. “It’s very similar to the size and scope of ‘Harry Potter.’” Amicucci said that the books are being closely monitored in distribution houses and stores across America.

Anticipation is high for Watchman because the existence of a To Kill a Mockingbird follow-up, an American classic, was a surprise to everyone around the world.

But is it the last book from Lee, the 89-year-old author who has only published one book until tomorrow? Apparently not, says Lee’s lawyer Tonja Carter, who’s believed to be responsible for Watchman seeing the light of day.

In an op-ed released on the Wall Street Journal’s website this morning, Carter says she recently went back to the safe deposit box where she discovered the Watchman manuscript. This time, she found more interesting material which is currently be investigated by researchers. “What we found was extraordinary and surprised even me,” Carter wrote in the op-ed. Not only did she find the original draft of To Kill a Mockingbird and the Watchman manuscript, but she found a “significant number of pages of another typed text.”

“Was it an earlier draft of Watchman, or of Mockingbird, or even, as early correspondence indicates it might be, a third book bridging the two?” Carter teases.

The team investigating the new documents will soon decide whether or not there’s a third book in these new pages under the “direction” of Lee.

Carter has clearly been eager to publish Lee’s previously-unreleased work since the author’s previous caretaker, Lee’s sister Alice, died last year. Don’t be surprised if a three-quel shows up in stores in the not-so-distant future.

You can read the first chapter of Go Set a Watchman, which is already causing controversy, right here.