Jake Gyllenhaal says his nieces inspired kids' book about what it means to be an aunt or uncle

Oscar-nominated actor Jake Gyllenhaal may be best known for leading roles in blockbusters like “Donnie Darko," "Brokeback Mountain” and “Nightcrawler," but to his two beloved nieces – Ramona and Gloria – he is simply “Uncle Jake.”

Gyllenhaal says he loves being an uncle to the two girls, ages 16 and 11, who are the daughters of his sister Maggie Gyllenhaal and her husband, Peter Sarsgaard, who are fellow actors.

Now, his nieces are inspiring his latest act as the author of a new children’s book, “The Secret Society of Aunts and Uncles,” which is co-authored by Greta Caruso and illustrated by Dan Santat.

The 48-page book is billed as a “madcap, magical adventure into the heart of what it means to have an aunt or uncle.”

Gyllenhaal spoke to ABC News’ Wil Ganss ahead of the book's Sept. 5 release about how his nieces inspired him to write it.

WILL GANSS: Why did you decide to write this book?

JAKE GYLLENHAAL: I mean, first of all, I love my nieces. They're just incredible humans. And when my first niece was born, you know, I was sort of like, “I'm becoming an uncle. Like, oh, I get to be Uncle Jake forever!” And then I was like, “What does that mean?” You know?

Particularly because she was an infant. But I think what I started to learn was it really meant whatever she wanted it to mean, you know? That became kind of what me being an uncle was and that it's individual to all of us. But I just loved the idea, also, that to write a book about a relationship that there aren't really any books written about.

GANSS: I love it. And the dedication in the book is “Ramona and G.G., Love Uncle Jake.” What is your relationship like now with your nieces?

GYLLENHAAL: I mean, my oldest niece is 16, you know, and my other niece is 11. They're both, you know, they’re like I said, they're extraordinary human beings. My sister, my brother-in-law have raised really great girls.

For me, being there for them in whatever way I can, acknowledging the things they're going through, both good and bad. To know also that as they grow older, I'll be someone they can come to if they ever need any outside advice, even with their parents, which you know is a really important position for an uncle or an aunt to be for their niece or nephew, so that they can grow and go through their own secret society of aunts and uncles and become great aunts themselves one day.

GANSS: What was that collaborative process like between both Greta and your illustrator? Because, you know, my first read, I was just paying attention to the words and then second read, I was just like paying attention to the pictures, the illustrations. What was that collaborative process like for you guys?

GYLLENHAAL: I mean, the illustrations are the words in a lot of ways for me. It is cinematic to me, and that's what I realized was, I really loved writing a children's book, because it is a visual medium.

I mean, it is literary and writing the story and writing the words and getting them down to just the necessary was really difficult and really fun, but the illustrations were my favorite part. They’re clues all over it, and they're little Easter eggs everywhere. And that's the kind of stuff I loved when I was a kid – was on the fifth, sixth, 15th, 25th, the 100th reading, you find a new little thing.

And as a kid, I think that's what we love. And the kids in us love that, too.

GANSS: In the book you write, “Bedtime is always three minutes before Mom and Dad get home.” Is that an Uncle Jake approved rule?

GYLLENHAAL: Absolutely. I think it's essential. I think, like, one of the most fun things I can remember as a kid was staying up.

GANSS: We know what WWJD. is. I want to play a game. WWUJD – What would Uncle Jake do? I'll give you a situation. As the official uncle-ology expert, I need you to weigh in on the following situation. You've literally written a book about it.

GYLLENHAAL: Yes, but I’m yet to be the expert.

GANSS: WWUJD? Your nieces want to have a tea party, but the big game starts in 10 minutes? WWUJD?

GYLLENHAAL: Tea party and game.

GANSS: Both?

GYLLENHAAL: Yeah! Let’s have tea and watch the game.

GANSS: Your nieces are in the car, and you’ve got the AUX cord. What song do you put on for the perfect carpool karaoke jam sesh?

GYLLENHAAL: Oh, well my nieces – it’s probably changed in the last week – but you know Sia is always a good go-to.

GANSS: Swing from the chandelier, baby.

GYLLENHAAL: Absolutely.

GANSS: Christmas is coming up. How do you go about finding the perfect gift?

GYLLENHAAL: Oh, you just listen all year.

GANSS: You play the long game. I love it. Your nieces want something sweet, but dinner is in half an hour. WWUJD?

GYLLENHAAL: Dude, we going to get something sweet. I don’t know why I said it like that, but yeah. That’s what we’re going to do.

GANSS: Sweet!

GANSS: The last thing I want to say, I literally found out like last week, I'm going to be an uncle for the first time. My sister and her husband are expecting.

GYLLENHAAL: No way, whoa! Congratulations.

GANSS: I haven't even told my best friends yet. What advice do you have for a first time uncle?

GYLLENHAAL: Probably the best advice I've gotten in a lot of things, which is just listen. And always add a little healthy dose of danger. A little “HDD.” That's in the book.

GANSS: Healthy dose of danger.

GYLLENHAAL: You know, you gotta be the one – you know, mom and dad are always going to be mom and dad. But, you know, you'll always be that outside source that can, you know, encourage safe risk and pushing them to grow in a different way. And hopefully you can be that. Good luck to you! It’s great fun and actually super easy!

GANSS: Thank you.