If you grew up loving books like Holes and Sideways Stories from Wayside School, get ready — beloved children’s author Louis Sachar is publishing his first novel for adults.

PEOPLE can exclusively share the cover of Sachar’s adult debut, The Magician of Tiger Castle, forthcoming from Ace next year.

In the new modern fantasy novel, Princess Tullia, who lives in the kingdom of Esquaveta, is set to marry a prince from a neighboring village in order to form an alliance. When the princess falls for a lowly scribe instead, her father the King enlists the help of a magician named Anatole, who must decide if he wants to betray the kingdom or the princess’s trust in the process.

Sachar tells PEOPLE in an email interview that The Magician of Tiger Castle was originally going to be a young adult novel.

“That was my initial premise, but I never stick to a rigid outline,” Sachar says. “Rather, I let the story and characters take me in different directions … I was in the middle of the third draft, when it became clear to me that Anatole had nudged his way into the center of the story.”

“Well, it’s not a good idea to have a 40-year old-man, desperate to save his career, as the main character of a book for young people, so I decided to refocus and turn this into a novel for adults — though perhaps those adults who still had a place in their hearts for the Wayside School books, or Holes,” Sachar adds.

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The author also shares that he was inspired by William Goldman’s The Princess Bride, as well as the Renaissance.

“People had to figure things out on their own, without the aid of computers or even standardized weights and measures,” Sachar said of the time period. “Every item was handcrafted. During the four years it took to write the book, I enjoyed escaping into that world. I hope the reader will too.”

Sachar’s readers may be surprised that the author is now venturing into fantasy, but the author says that he doesn’t “think in those terms.”

“I try to write intriguing, fun and thought-provoking stories with engaging characters,” Sachar says. “Whether others call my work fantasy, absurdism, surrealism, magical reality or something else is unimportant, just as long as the reader keeps turning the page.”

The author also shares that The Magician of Tiger Castle represents a milestone for him, as well.

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“I think I’m more excited about the publication of this book than I had been for any other book I’d written, except for probably my first book, Sideways Stories From Wayside School,” Sachar says. “I was 23 when that book was published. I’ll be 71 when The Magician of Tiger Castle comes out.  By then, as Pito tells Anatole, the arrow will have left my bow.”