One day in the late 1960s, a customer entered Justin Schiller’s Antiquarian Book Center in Rockefeller Center’s subterranean shopping arcade, wanting to see Schiller’s collection of 19th-century pop-up books by the German artist Lothar Meggendorfer. Thus began a business relationship that blossomed into a friendship lasting 45 years, until the customer’s death last year.

That customer was Maurice Sendak.

At the time he met Schiller, Sendak was just developing a fondness for rare books and ephemera, which Schiller nurtured. Sendak loved Herman Melville (Schiller helped him buy a first printing of Moby-Dick), Mozart (a handwritten letter), and German romanticism. A four-page letter handwritten by Wilhelm Grimm that contained an unpublished fairy tale came into Schiller’s shop and became the basis for Sendak’s 1988 picture book Dear Mili.

“Justin would acquire something for Maurice but instead of taking cash, often he’d be paid in Maurice’s artwork,” said Leonard S. Marcus, the children’s book historian. As a result, Schiller amassed an “amazing collection of hundreds of pieces of Maurice’s work. I think it’s safe to say that no other private collection touches his, or even comes close.”

Beginning June 10, on what would have been Sendak’s 85th birthday, that collection will go on display at the Society of Illustrators in New York. The exhibit, containing more than 200 previously unpublished studies, sketches, and ephemera, will run through August 17. Every piece is from Schiller’s collection.

“I think the most stunning works are the etchings, which I’d never seen before, some of which were based on Where the Wild Things Are, and others made in relation to Outside Over There, which are ravishingly beautiful,” Marcus said. “Apparently, [Sendak] considered using etchings to illustrate Outside Over There. That there was this whole other art form that he was deeply immersed in came as a complete surprise to me.”

Abrams will publish the exhibit’s catalogue, Maurice Sendak: A Celebration of the Artist and His Work, written by Schiller and David M. V. Dennis, and edited by Marcus, with more than 200 illustrations and essays about Sendak’s work from Iona Opie, Paul O. Zelinsky, and others. Abrams editorial director Charles Kochman, who acquired the book, says it stands apart from other collections of Sendak’s art because of its breadth. “The range of material is really revelatory because the work shows what a wide swath he cut in the field, not just picture books, but advertising, theatre, posters, work for adults. We’ll be selling it passionately to all of Maurice’s many admirers.”