When Jake Gyllenhaal first became an uncle, he was admittedly “overeager.”

“That was the first experience I’d ever had with being given the uncle title, and so I took it very seriously,” he told Parade.

That was almost 17 years ago, when his sister, Maggie Gyllenhaal, gave birth to her oldest daughter, Ramona (her youngest, Gloria, is 11). Now, the Nightcrawler actor is paying tribute to his “sacred” role with a new children’s book, The Secret Society of Aunts and Uncles, which he co-wrote with his longtime friend Greta Caruso (daughter of actor David Caruso).

The charming story follows 10-year-old Leo as he watches his ordinarily boring Uncle Mo get a crash course in “Uncleology.” (Lesson No. 1: Bedtime is not 8 o’clock. Lesson No. 2: Broccoli comes after ice cream.)

Gyllenhaal and Caruso initially thought to center their book around a “perfect” uncle. But over time, “we realized the much more interesting story was about someone who was an uncle but hadn’t really learned [the ropes yet]. People aren’t just magically great aunts and uncles,” Caruso told Parade. “It requires its own craft, skills and technique.”

Below, Gyllenhaal and Caruso, who’ve been “best friends” for about 20 years, discuss Jake’s babysitting skills, growing up famous and the mystery of cotton candy.

Greta, you have a new baby, is that right?
Greta: I do. She’s 11 months. I’m really exhausted today. I’m slumped over in this chair.
Jake: She’s like, “This chair is really comfortable” [pretends to pass out].
Greta: We were truly finishing this book while I was in the hospital giving birth.
Jake: That’s true. The last week of us finishing, she was non-responsive. She’s like, “The book’s great. I’m fine. I’m good with whatever you decide.”

Jake, are you jumping in to babysit?
Jake: I haven’t yet babysat, but we hang out a lot.
Greta: He’s fully Uncle Jake.
Jake: I should add, her baby is pretty cute. I was pretty moved in the first few days after Greta gave birth that she trusted me so much. [To Greta] I remember I was holding her, and then you left [to go upstairs]. You had three days of not being able to go to the bathroom.
Greta: I came downstairs and Jake was like, “Hello? Hello? I have the baby.”

Jake, have you gotten better with infants since your nieces were that young?
Jake: Yes. When my nieces were very young—I mean, I really loved taking care of my sister in the early days right after she had given birth, particularly to my first niece [Ramona, now 16]. That was the first experience I’d ever had with being given the uncle title, and so I took it very seriously and showed up and was overeager for the first few months. There was nothing to do, really.
Greta: No, just putter around, clean surfaces.
Jake: I cooked for my sister a lot in those early days when she was just out of the hospital, and that was really [my way of] feeding Ramona at that time too. I was excited about that. I love babies.

How do you like being an uncle to a teenager and an 11-year-old? That’s very different from being an uncle to babies.
Jake: Oh, absolutely. I can’t tell you how proud of them I am. They are, both of them, just extraordinary humans, and that’s a testament to their parents. My sister and my brother-in-law [Peter Sarsgaard] are just curious, open, encouraging, and I think they’ve helped create or helped these girls create themselves. I’m just so proud of them. The fact that Ramona is 16 blows my mind. But more than that, just because time is so trippy, I think who she’s become is what blows my mind. She’s just so engaged, so smart, and just a beautiful human. Then Gloria also is just… She’s an incredible musician, and they’re just both curious. To me, that’s the most important human quality, is curiosity, and they both have it in droves. Being their uncle, I’m just so psyched that I’m a part of their life.

Do they think you’re cool?
Jake: No.

But you’re a movie star! Whereas Uncle Mo in this book is going to a rubber-band convention.
Jake: Yeah. He’s a stationary supply salesman. We googled at some point, “The most boring job ever.” We decided that it sounded pretty boring.
Greta: I mean, we just wanted Mo to be very passionate about what he was passionate about and [have] it also be something that a [young] boy would not be particularly inspired by, although I will say, when I was 9, I was obsessed with stationary supplies.
Jake: Yes. The child in me to this day is obsessed with stationary.

You both grew up with fame around you. I’m curious how aware one can be of how different that is, how special. Was it something you understood at the time?
Greta: I wasn’t aware at all. It was just in the water around me. I also think, because I’m a million years old, it was the ’90s. It was a very different time. Fame operated in a very different way. So I think that is pretty salient. But, no, it wasn’t really that apparent. I look back on it, and I’m like, “Oh, that was untraditional in some ways.” Both of my parents were very present, so it didn’t feel weird.
Jake: What I’ll say is that the most important thing to me has been my family and my friends throughout the years. That’s it. Whatever anybody does in their life, the barometer for my interest is whether or not we can connect, and it’s always been that way, and that’s what I was taught, and that’s what matters to me.

And when you look at your nieces or any other young people in your life who are similarly surrounded by fame now, how do you help guide them through that experience, if at all?
Jake: Oh, I think they’re fully capable, and I think my job and the people around them who have chosen to be in the public eye, that’s not their responsibility, it’s ours.

What do you mean, it’s not their responsibility? What isn’t?
Jake: Meaning that it hasn’t been their choice. So, to me, even when I was writing the book, Uncle Mo is named after Ramona, and the great-aunt, Gloria, is [named after] my other niece. Leo is my dog’s name. So there are little Easter eggs all over the place, and I wanted to make sure they were comfortable with that idea before we put that in a book in the world. I wanted to know if my sister was and they were, because it’s their choice and our relationship is ours. It’s nobody else but ours. They gave me the permission and were really excited about it. So that’s what I mean.

Did your godmother, Jamie Lee Curtis, give you any advice on writing a children’s book? She’s a pretty prolific children’s book author herself.
Jake: She’s an extraordinary children’s book writer, and her children’s books rhyme, which is—
Greta: That’s some elite varsity stuff. Early on, we were like, “Should we rhyme?” Our editor was like, “Let’s just start”—
Jake: I was like, “Should we dunk?” They’re like, “You know what? Just stick to layups. It’s OK.” But she did [give advice], actually. She read a very early version of the book and noted it, and it was so, so helpful. I think primarily helpful, like any good artist, in saying, “Move towards what’s most true.” That’s what she seems to do in her life. She’s a really honest person, and she’s really become, I think, at least in the public eye, a real legend as a result of that. She’s always been a legend to me. There’s nothing that people are learning about her in the world now that I haven’t known about her for many years. She’s straight, she’s thoughtful, she’s clear about things, and she’s just… Again, I keep using this word, but she’s just endlessly curious and prolific in what she does as a result of that, and I just admire it so much.

Since it’s come up a couple of times, what are some things you’re curious about right now?
Jake: Curious question.
Greta: It’s been really interesting to have a kid of my own and look at children’s books from a totally different perspective, because I just read so many of them every day—just constantly, 24-hours-a-day.
Jake: I’m curious about cotton candy, but I’ve always been curious about cotton candy. It’s a baffling experience. I still to this day am in awe of it and enamored by it.
Greta: It just dissolves.
Jake: No, but I just finished reading Mark Harris’ book about Mike Nichols, which I was a bit late to the party on, but I really loved it. I’ve been more interested now in biographies. I was just recommended to read the Rockefeller biography by [Ron] Chernow. That is going to be quite a journey. It’s heavy, and it’s going to really sit in my backpack for a long time.

The Secret Society of Aunts and Uncles is out Sept. 5.