October 25, 2013 04:00 PM

John Lithgow‘s Never Play Music Right Next to the Zoo may be his ninth children’s book, but it’s by far his favorite — and longest — adventure to date.

The story, which tells the tale of a concert group who set up shop next to a zoo full of curious critters that break free to get their hands (paws!) on the instruments, was originally a musical co-written by Lithgow nearly a decade ago.

“The words of this book are the narration of a musical suite I created with a wonderful composer named Bill Elliott. We’ve been performing it ever since,” the actor, who will hold a Celebrity Storyteller book signing on Saturday at McNally Jackson in New York City, tells PEOPLE.

Courtesy Simon & Schuster Kids; Inset; Jim Spellman/WireImage

Although Lithgow’s kids did not make up the book’s fan base when he first created the musical (“my own children were too old for it”), he admits that the arrival of his grandchildren has been a game changer.

“My grandchildren are 5 and 8 years old and they adore the book. I read it to them a week ago for the first time,” he shares. “To my astonishment, the very next day, my granddaughter was able to recite the whole story to me.”

But this isn’t their first taste of his incredible talent. “My grandkids are quite aware of me — and at the moment they’re very proud of me,” Lithgow, who currently is the voice of the White Rabbit in Once Upon a Time in Wonderland, tells PEOPLE.

He jokingly adds, “It’s only in a few years when the embarrassment factor kicks in.”

— Anya Leon with reporting by Caryn Midler

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