Charles Santore was in the middle of illustrating the children’s book he did not know would be his last when he began to feel weak.

The book was “The Scroobious Pip,” Edward Lear’s nonsense poem about an uncategorizable creature: part beast, part bird, part fish, part insect. The man bringing it to visual life was a beloved illustrator, a master of realism whose versions of “The Night Before Christmas,” “Peter Rabbit” and “The Wizard of Oz” have sold hundreds of thousands of copies.

Over the course of his career, Charlie, as he was known, rarely missed a day in his studio, two blocks from Rittenhouse Square in Philadelphia. But suddenly, he found himself in so much pain that he was unable to work.

On Aug. 11, 2019 — only six days after he was admitted to Pennsylvania Hospital — Charlie died. He was 84.

Soon after, his friend and agent Buz Teacher called a meeting with Charlie’s three adult children to discuss their father’s work. Among the most pressing questions was how to proceed with “The Scroobious Pip,” which was under contract with Running Press, a Philadelphia-based imprint of Hachette. Charlie had made nine drawings for it — each one an incredibly detailed menagerie — and three watercolor paintings. But there was an enormous amount of work left.

Charlie’s daughter Christina had an idea. What if her younger brother, Nicholas — Nicky — finished illustrating the book? After all, Nicky was the Santore who had followed most closely in his father’s footsteps: He’d gone to the Rhode Island School of Design and then to Yale for an M.F.A. in painting. According to those around him, Nicky really had a gift.

Charlie’s youngest brother, Joe Santore, a fine artist who teaches at the New York Studio School, recalled Nicky’s early drawings as “very impressive, very quiet — like him, very beautifully drawn and gentle.” “There was a beautiful light in them, a real feel for the quality of line and the touch,” he said.

Nicky’s first reaction to the suggestion that he complete “The Scroobious Pip” was skepticism. He didn’t know if he could do the project justice. Even if he could, the role of art in his life had long been a source of tension with his father.

Growing up, Nicky admired his father’s skill. “I remember always smiling when he would draw something, because he was so good,” Nicky said. “He was such a good draftsman.” As a child, Nicky took to drawing quickly, eagerly completing visual exercises his father assigned him.

But despite his talent, Nicky had many other interests. After his first year at Yale, he spent his summer at home, surfing and playing music. His father disapproved. Charlie was a perfectionist and a professional, someone who would never miss a deadline. He was deeply focused on his art, family members said, and he couldn’t understand why Nicky, who had such obvious artistic talent, wasn’t tending to it.

The day Nicky was due to depart for New Haven, Conn., Charlie took him aside for a talk. “You need to focus if you want to be serious,” Nicky recalled his father saying. But for a very long time, Nicky resisted. “Our ideals were at odds. … It turned me off, in a way. It wasn’t what I wanted to hear.”

By the time of his father’s death, Nicky had been drifting away from visual art for nearly a decade. He had participated in a couple of studio shows, but had lately become something of a jack-of-all-trades, taking on carpentry work, playing in a band and helping to raise his two young daughters. In other words, doing a little of everything — except painting.

Aside from the philosophical disconnect between father and son, two technical differences separated Nicky’s art from Charlie’s. For his children’s books, Charlie had mainly used watercolor — a notoriously unforgiving medium — whereas Nicky mostly painted in oil. And if Nicky’s relationship to visual art in general was fraught, his relationship to the art of illustration was even more so.

At Yale, Nicky had been encouraged to move away from anything deemed commercial. His education was unlike the one his father had received, which amounted to “draw well and you’ll be a good artist,” Nicky said. After Yale, Joe recalled, Nicky’s “work became much more geometrically oriented, structurally oriented.” It caught the attention of some gallerists; Nicky now feels he might not have made the most of the opportunities that arose.

“I’m bad at follow-through,” he said.

And so, characteristically, Nicky assumed the question of whether he could finish “The Scroobious Pip” was one he could return to after he had more time to think.

But two days after their initial meeting, Buz called again. He had mentioned the idea to Running Press — which Buz and his brother Lawrence founded in 1972, before selling it in the early 2000s — and had received an enthusiastic response.

“I was like, well, wait,” Nicky said. “You haven’t even seen anything I’ve done. I don’t even know if I could do it.”

He decided to spend several months in his father’s longtime studio, surrounded by Charlie’s art, files and photo references. There, he would try to dust off the dormant technical skills he had developed as a younger artist, including some he had learned directly from his father. Running Press would take a look at the results whenever Nicky felt ready; collectively, they would agree on whether to proceed with “The Scroobious Pip.”

The pressure on Nicky to live up to the family name came not just from his father. In some ways, it could be said to come from the city of Philadelphia, where, in certain circles, the Santore name is renowned.

Charlie’s father, another Charles, was a boxer and union organizer who now has a branch of the Free Library of Philadelphia named in his honor. That Charles and his wife, Nellie, had four sons: Charlie was the oldest; then came Bobby and Richie, twins who founded the Saloon, a fabled Philadelphia restaurant (worth visiting for its décor alone, which was largely overseen by Charlie); and then came Joe, the contemporary artist.

The next generation proved equally interesting: Nicky’s oldest brother, Charles III, is — almost unbelievably — a professional safecracker; his sister Christina is a writer and editor who lives in Amsterdam.

Looking at the highly varied accomplishments of the Santores, one might imagine a sort of Philadelphian version of the Royal Tenenbaums: children of privilege, or at least of intellectuals. But the four Santore brothers and their descendants, according to Joe, were pulled toward creative fields not because of their upbringing, but in spite of it.

Their part of Philadelphia — now called Bella Vista and then known by its parish name, St. Mary’s — was “kind of a wild neighborhood,” Joe said. And Charlie was a neighborhood guy. Known for his street fighting and his pool playing, “he didn’t take any nonsense from anybody.” “But on the other hand, he was interested in art, music,” Joe said.

Asked whether it could have been their parents who encouraged the Santore brothers creatively, Joe thought for a bit. “It wasn’t my dad who was interested in art,” he said, though their father was proud of their abilities and took commissions from the neighborhood for his sons’ hand-drawn Christmas cards. Nor, Joe said, was their mother, though she was known to draw a little.

Mainly, he credits two Philadelphia public schools: the James Campbell School, where they went for their elementary education — “it was the kind of school that encouraged you to do what you were good at,” Joe said — and Edward Bok Technical High School, where Charlie studied design. When Charlie was awarded a full scholarship to the Philadelphia Museum School of Art for an undergraduate degree, it was a big deal. “Nobody went to college,” Joe said.

But Charlie did. It would become the first step toward a career in art that included a long chapter in commercial illustration, a lifelong passion for antiques (he wrote the definitive texts on Windsor chairs) and ultimately a career as a children’s book illustrator. And Charlie’s higher education would become an important step for the rest of the family, too: It was Charlie who pushed Joe, then two years out of high school and feeling adrift, to consider a college degree. “He said to me, ‘What are you doing with your life?’” It wasn’t long before Joe was enrolled at the Philadelphia College of Art.

In life, and even after death, Charlie seemed to have a way of bringing the other artists in his family back to the work he felt sure they should be doing. Some of his final words to Nicky had been: “Just paint. You’ll find your way.”

So in 2020, Nicky sat down in Charlie’s studio, regarded his father’s work in progress and set out to do exactly that — paint, but not without trepidation.

“The first meeting we had with him, he looked very nervous,” said Julie Matysik, editorial director of Running Press Kids.

Frances Soo Ping Chow, vice president and creative director of Running Press, offered some simple advice. “You don’t have to live up to anyone,” she said. “This is your project now.”

The work was slow at first. For Nicky, it was challenging to abide by his father’s singular rules. Any white in the picture had to be the white of the paper: Charlie thought it was cheating to add white paint after the fact.

Nicky took nearly three years to finish the book. But by 2023, the artwork for “The Scroobious Pip” was complete — and remarkable. On one page, the translucent wings of a dragonfly refracted the green of tall grass in the background. On another, sea creatures breached a blue-and-gray ocean.

“It’s been amazing to watch,” Matysik said.

Next to her, Soo Ping Chow looked over Nicky’s finished portfolio in the offices of Running Press. It was possible to discern a slight difference in style between Charlie’s three paintings and the rest, which were Nicky’s: Charlie’s palette was brighter, Nicky’s more subdued; Charlie’s technique was dryer, Nicky’s more liquid. But rather than feeling accidental, the effect seemed intentional, and moving. A son and his late father, still and always in conversation with one another: There was something nearly supernatural about it.

“You can see it,” Soo Ping Chow said. “This book is really beautiful.”

Did Nicky agree? Characteristically, he hesitated. “I think we pulled it off,” he said, at last.

As for what’s next: Nicky is making his own paintings again. He is also at work on another children’s book for Running Press, “The Three Witches,” inspired by MacBeth and by Henry Mercer’s Tile Works in Bucks County, Pa. He is returning to the geometric style that characterized his solo work; he is also using the lessons he learned from finishing “The Scroobious Pip.” For “The Three Witches,” he will use watercolor again, he said — the medium his father loved so much.

Only this time, Nicky said, he’ll do it his way.