Prolific author, illustrator, and educator Jean Marzollo, widely known for the bestselling I Spy series, died in her sleep of natural causes on April 10 at her home in Cold Spring, N.Y. She was 75.

Jean Marzollo was born Jean Martin on June 25, 1942 in Manchester, Conn., the youngest of three children. In her 2007 essay for Something About the Author, she recalled that her childhood was happy and active, spent in a neighborhood filled with children, many of whom became lifelong friends, and featuring a slope perfect for sledding and riding bikes. Though she had not yet kindled an interest in writing, Marzollo said she discovered an early passion for making things with her friends, especially doll clothes. “I never thought about being an author when I was young, but the pleasure I take now in making books is the same pleasure I took making doll clothes as a child,” she wrote.

In 1960, Marzollo graduated from Manchester High School and had set her mind on becoming a social worker with a goal of helping poor people improve their lives. She enrolled at the University of Connecticut, focusing on home economics and hoping to put her sewing skills to good use toward her career aspirations. It was a summer trip abroad in 1962 with a group called Experiment in International Living that helped Marzollo expand her world view and inspired her to change her major to English so she could read more deeply about different cultures. In the middle of her senior year at UConn, Marzollo was accepted to, and was able to begin, studies at Harvard Graduate School of Education in Cambridge, Mass., having amassed enough credits to leave college early. She earned a Masters in Arts of Teaching in 1965 and began teaching high school English in Massachusetts.

Marzollo’s desire to aid poor people continued to burn bright and in 1967 she became assistant director of Harvard’s Upward Bound summer program, designed to help at-risk high school students succeed. The experience and its challenges spurred Marzollo to move to New York City later that year in hopes of publishing educational materials that could help the kinds of children she met through Upward Bound.

Once in New York, Marzollo found a position creating preschool materials, first for the General Learning Corporation, and then as a freelance writer. Around this time, she met sculptor Claudio Marzollo, who became her husband in 1969. Marzollo’s freelance career flourished, and she soon founded a small company with two friends—called Education Workshop—and wrote the first parent-teacher guide to Sesame Street, which had debuted on PBS in 1969. In 1970, Marzollo was named director of publications for the National Commission on Resources for Youth. She continued to write about family and early childhood issues, publishing articles in such magazines as Parents and Redbook.

By 1972, Marzollo had taken on the position of editing Scholastic’s monthly kindergarten magazine, Let’s Find Out. Her first published book for adults, Learning Through Play (Harper & Row), which was informed by her magazine work, was published that same year. Marzollo has said that she found her work on Let’s Find Out so fulfilling that she kept the job for 20 years.

As Marzollo tells it, the idea for her first children’s book also grew out of her experiences at Let’s Find Out. She was a new mom (son Daniel was born in 1973) when she began jotting down some rhymes inspired by a humorous drawing by illustrator Irene Trivas. Once she finished the poem, Marzollo showed it to another illustrator, Susan Jeffers, who had recently worked on a project for Let’s Find Out. Jeffers went on to show the text to her editor at Dial, Phyllis Fogelman, who wanted to publish it. Close Your Eyes, illustrated by Jeffers, was released by Dial in 1978, and dedicated to Marzollo’s second son, David, who was born in 1975.

Marzollo’s most widely known projects were the bestselling I Spy seek-and-find riddle books, which she created in collaboration with photographer Walter Wick. In her Something About the Author essay, Marzollo related that their creative partnership also emerged from her experience at Let’s Find Out. Wick had mailed a promotional card to the magazine, and as soon as Marzollo saw it she “knew that this photographer would be perfect for kindergarten because his picture was so clear and enticing.” She hired him to create a series of photos and posters with various themes and objects that interest children.

The photos caught the eye of Grace Maccarone, then senior editor, and Bernette Ford, then editorial director, at Scholastic’s Cartwheel Books imprint. That interest led to the first book, I Spy: A Book of Picture Riddles (1992), written by Marzollo with photos by Wick and art direction by Carol Devine Carson, who had also been working with Wick at Let’s Find Out. The collaborators created seven more classic I Spy books in an oversize picture book format, and the myriad spin-offs from the series—some co-created by Marzollo’s sons—have included easy readers, I Spy Challenger books, board books, and such merchandise as puzzles and electronic games.

In the late 1990s, Marzollo began painting with watercolors, largely for her own enjoyment. But when she was asked to write Bible stories for young children, she also became a children’s book illustrator, providing the art for her text. She created a series of Bible story retellings including Daniel in the Lions’ Den (Little, Brown, 2003), and several retellings of Greek myths all for Little, Brown, and she wrote and illustrated two counting books, Ten Little Eggs (HarperFestival, 2004) and Ten Little Christmas Presents (Scholastic, 2008).

Marzollo had another success with the Shanna Show series of early readers from Hyperion (2001–2004), a project suggested by then-editorial director Andrea Davis Pinkney. The books were adapted for TV as Shanna’s Show, an animated series that ran on Disney Channel’s Playhouse Disney from 2004 to 2010.

In all, Marzollo created more than 150 books for children. She maintained her love of working with and for children and families throughout her life. “Even though I am now old enough to retire,” she wrote in 2007, “I remain young at heart, perhaps because I write and illustrate for children.”

Jean and Claudio Marzollo moved from New York City to Philipstown, N.Y. (where the village of Cold Spring is located) in 1976 to raise their sons and were very active in their community, involved in library, literature and theater organizations as well as the local school board. In November 2017, the Butterfield Library in Cold Spring named its new children’s room after Jean and dedicated it to her.

Wick posted a remembrance of Marzollo on his Facebook page. In part, it reads, “I Spy sprang from a previous collaboration, its success was a complete surprise to both of us, and it changed our lives. It was Jean who recognized the educational potential of I Spy, and her ability to articulate those values made me a better illustrator. Her writing, so seemingly simple, was in fact carefully honed to provide young readers with the richest vocabulary-building experience possible. Simply put, the I Spy series would not be what it is without Jean Marzollo. She was a force in children’s book literature and early childhood education circles and will be sorely missed.”

Grace Maccarone, now executive editor at Holiday House, edited a variety of books by Marzollo since the first I Spy title in the early 1990s. In reflecting on their close relationship, she shared this anecdote: “As well as being one of my authors, Jean was a mentor and a dear friend who often gave me reassuring parenting advice. And she was profoundly creative. One day I called Jean because I wanted her to write nonfiction at a first grade reading level. After we discussed business, I told Jean about a conversation my six-year-old daughter and her friend had in the backseat of my car. Every time my daughter made a statement, the friend would one-up her. This inspired Jean to write an adorable easy-to-read nonfiction book called I’m a Seed, a dialogue between two seeds that imitated the girls’ conversation. In the end, the seeds discover they have become different plants—a pumpkin and a marigold. ‘There should be a name for it,’ the pumpkin says. ‘There is,’ replies the marigold. ‘It’s called life.’ I miss her.”