How to pre-order Rebecca Yarros' now-viral, 'Untitled Empyrean' book

• Author Rebecca Yarros announced a surprise "Untitled Empyrean" book, separate from the fourth book in the "Fourth Wing" series.
• The new book is scheduled to be released on Tuesday, Sept. 29.
• Fans pre-ordered the untitled book to the number one spot on Amazon without a cover or title.

Ready to head back to Navarre? "Fourth Wing" author Rebecca Yarros revealed a surprise over the weekend, sharing new details about her "Untitled Empyrean" book.

In a post on Instagram, Yarros said she came up with the idea about a year ago.

"But then you not only found it as soon as my publisher put it on preorder, but shot it straight to number one on Amazon without a cover, title, or even a vague idea of what might be coming your way!?!?" Yarros added. "You’ve rendered me speechless."

The now-viral "Untitled Empyrean" is scheduled to be released on Tuesday, Sept. 29, and Yarros has confirmed that the book is neither the graphic novel nor the fourth installment in the "Fourth Wing" series.

So for now, fans will have to wait a little longer to get answers after the major cliffhanger in "Onyx Storm." In the meantime, here's how to pre-order Yarros' "Untitled Empyrean" book today.

How to pre-order Rebecca Yarros' Untitled Empyrean book

Untitled Empyrean (Not Book Four) by Rebecca Yarros

Pre-order Yarros' "Untitled Empyrean" set to be released on Tuesday, Sept. 29. Pre-order now at Amazon

Catch up on Fourth Wing series

Fourth Wing Series book set

Haven't started "Fourth Wing" yet? Catch up on the series with "Fourth Wing," "Iron Flame" and "Onyx Storm." Get now at Amazon

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When is Rebecca Yarros' Untitled Empyrean book coming out?

Yarros' Untitled Empyrean is set to be released on Tuesday, Sept. 29.

When will Rebecca Yarros release Fourth Wing book four?

Yarros has not yet set a release date for the fourth book in the installment of her "Fourth Wing" series. Although, she has revealed that she has a playlist for the fourth book already, fueling even more fan theories and speculation about what could happen in the next book.

When will the Fourth Wing TV series be released?

There is not an official release date yet for the "Fourth Wing" TV series; production is still in the early stages. The series is being produced by Amazon MGM Studios and Michael B. Jordan's Outlier society.