AMSTERDAM — During World War II, a clutch of whimsical children’s books were published in the Netherlands under a pen name, El Pintor. One book shows children flying on the backs of sparrows. In another, they float, attached to balloons. There is a pop-up book with people and animals nestled in trees and an activity book with paper cutouts.

The books sold thousands of copies, and were popular not only in the Netherlands, which was invaded and occupied by Nazi Germany in 1940, but in Germany as well.

The books did more than entertain children during the grim days of war. Behind the pseudonym El Pintor was a Jewish couple, Galinka Ehrenfest and Jacob Kloot. They used the name El Pintor to obscure their heritage, and funneled the proceeds from their picture books to fund Dutch resistance efforts and to help Jews who were hiding from the Nazi regime.

They did so at great risk, said Linda Horn, who wrote a book published in the Netherlands about Ehrenfest’s life.

“Secrecy was very important, people couldn’t write down what they were doing,” said Horn of those who worked in the Dutch resistance. “There are barely any sources.”

El Pintor, which also included the work of other artists and writers who collaborated with Ehrenfest and Kloot, produced about two dozen children’s books and games in the early 1940s. Now, 23 of the books — including copies of all titles published in Dutch, one published only in German and several translations — will be presented for sale this week at the New York International Antiquarian Book Fair.

Peter Kraus, the owner of Ursus Rare Books, who is selling the collection, said a Dutch collector acquired it, bit by bit, over the course of 30 years.

Kloot came from a large, blue collar Jewish family in Amsterdam, while Ehrenfest was born in what is now Estonia. Her father, Paul Ehrenfest, moved the family to Leiden, in the Netherlands, in 1912. A prominent physicist, he was friends with Albert Einstein. According to an article that ran on the front page of The New York Times in 1923, Paul hosted Einstein when he fled from Berlin to Leiden, planning to stay “until general conditions improve and the anti-Semitic hatred subsides in Berlin.”

Ehrenfest spent a few years in the United States in the early 1930s, when she attended art school in California and started drawing regularly. She then returned to the Netherlands and enrolled in the New Art School in Amsterdam, which was founded by an artist who had fled Germany. The school was later forced to close by the Nazi regime, who remained in parts of the Netherlands until the German surrender in 1945.

It was at the New Art School that she met Kloot. They moved in together in 1936 and were married five years later.

In 1940, Kloot founded a small publisher in Amsterdam called Corunda, through which El Pintor started publishing children’s books. Ehrenfest became the creative powerhouse, drawing and writing stories, while Kloot managed the business.

Publishing books was difficult during the Nazi occupation. Paper was scarce and expensive, and printing books required official permission. Approved books got a serial number which allowed their publication, their sale at bookstores and, in El Pintor’s case, their export to Germany.

In 1941, the Nazi regime forced Jewish businesses to be given over to non-Jews. Kloot did so, handing it over to someone he knew, but stayed involved in the operations.

Kraus, who is selling the El Pintor collection, said part of what makes the books unique — aside from the powerful story behind them — is their variety. There are picture books, activity books and early chapter books. Some of them are roughly the size of an adult’s hand, while others are much larger, like a thin coffee table book. Horn said they were all meant to make children think and play differently than other, more traditional books of the time.

“The books encourage kids to make a mess, to draw on white walls, those sorts of funny things,” Horn said.

Though thousands of copies were printed, Kraus said very few remain — perhaps because they are books for children. “Children’s books tend to be rare,” he said, “because children ruin them.”

As the war continued, Kloot and Ehrenfest became deeply involved with the resistance and helped people escape Nazi persecution. Kloot often traveled around the country, helping those in danger find places to hide.

In 1943, Nazi officers arrested Kloot and his business partner in Leiden. They let the partner go, but Kloot, who was 26, was deported and sent to Westerbork, a transit camp in the Netherlands, and from there to Sobibor, an extermination camp, where he was murdered.

At the time of Kloot’s arrest, Ehrenfest was pregnant with their first child. Shortly after, she gave birth to a stillborn baby. She tried to continue producing books as El Pintor, Horn said, but ultimately, it proved too difficult without Kloot and with the growing dangers and challenges of the war years.

Ehrenfest survived, and published one last book as El Pintor after the war ended. She stayed in the Netherlands, where she died in 1979. She was 69 years old.

“It’s a ghastly moment in history, and it’s a paradox that this horrible thing would have such an aesthetic memorial,” Kraus said, gazing at El Pintor’s books, the colors still vibrant after 80 years. “At least this man, this couple, is remembered.”