It’s clear that readers still have Fifty Shades fever. The fourth book in the series, a spinoff of the original titled Grey, has sold a tantalizing number of copies.

Grey publisher Vintage (an offshoot of Penguin Random House) announced on Monday that the book has sold 1.1 million copies since hitting book store shelves on Thursday, June 18 — Christian Grey’s fictional birthday.

“Christian’s side of the story is proving to be irresistible,” said Vintage’s Anne Messitte (via Publisher’s Weekly).

The new book, which is Fifty Shades of Grey but from Christian’s perspective instead of Ana’s, is still very far from nearing the success of the other books in the trilogy. The first three books in the series have sold over 125 million copies worldwide since the first book was released in 2011.

It’ll be interesting to see how Grey continues to sell in the months ahead. The series has so far benefited from a ton of word of mouth (both good and bad) as the world became seemingly enamored with the idea of erotic writing within a book you can purchase at the front of Barnes & Noble. Grey’s success will likely ride in part on how many readers tell their fellow Fifty Shades-loving friends about how much they liked (or disliked) the book.

If the book continues to sell well, E.L. James could very well decide to publish spinoffs of Fifty Shades Darker and Fifty Shades Freed. With Universal already working on film adaptations of the next two books, the fandom will continue to thrive over the next few years, thus other books are quite possible.

Fans must stay on their toes — Considering how quickly Grey went from announcement to release (17 days!), we could see additional books in the series arrive in the future without much notice.