NEW YORK, NY.- As Mark Eden Horowitz, a senior music specialist at the Library of Congress, was digging through playwright Neil Simon’s manuscripts and papers this year, he made a surprising discovery.

Simon, the most commercially successful American playwright of the 20th century, could also draw. Like, really draw.

“They’re almost professional,” Horowitz said in a recent phone conversation of some of the pen-and-ink drawings and paintings he found tucked among the scripts. “There are two watercolors in particular that are quite beautiful landscapes.”

More than a dozen notepads filled with drawings, cartoons and caricatures by Simon, who died in 2018, was just one of the surprising discoveries Horowitz made in the trove of approximately 7,700 of the playwright’s manuscripts and papers (and even eyeglasses), a collection that the library on Monday announced had been donated by Simon’s widow, actress Elaine Joyce.

An event Monday at the library in Washington, which will stream live on its YouTube channel at 7 p.m. Eastern, will include a conversation with actors Matthew Broderick and Sarah Jessica Parker, who are starring in the Broadway revival of Simon’s 1968 comedy “Plaza Suite,” as well as remarks by Joyce.

The collection includes hundreds of scripts, notes and outlines for Simon’s plays, including handwritten first drafts and multiple drafts of typescripts — often annotated — as well as handwritten letters to luminaries such as August Wilson. There are more than a dozen scripts (sometimes many more) for some of his most celebrated shows, including “Brighton Beach Memoirs,” “The Odd Couple” and “Lost in Yonkers,” Simon’s dysfunctional-family comedy that won a Tony Award as well as the Pulitzer Prize for drama in 1991.

Sometimes, Horowitz said, it took some detective work to identify a famous play, which existed in an early version under an alternate title. (An early script for “Lost in Yonkers” has the title “Louie the Gangster,” and “Brighton Beach Memoirs” was once “The War of the Rosens.”)

“Sometimes you’re not sure when you open the title and then you realize, ‘Oh, this became that,’” he said.

The collection includes materials from the 25 screenplays Simon wrote, including “The Prisoner of Second Avenue,” “The Heartbreak Kid” and “The Goodbye Girl,” for which he won a Golden Globe in 1978. There are also several scripts for shows never completed or produced, such as one titled “The Merry Widows,” written for Bette Midler and Whoopi Goldberg, and “A Foggy Day,” a musical that uses the songs of George and Ira Gershwin.

“Every time you open a carton, it’s like, ‘Oh, my God, what’s going to be in here?’” Horowitz said.

Beyond dozens of unknown works in progress — some comprise just a few scenes, while others have multiple drafts — the archive also includes Simon’s Pulitzer Prize, his special Tony Award and at least two Golden Globes, as well as photographs, programs, original posters and even baseballs signed by several Hall of Famers, among them Tommy Lasorda, Eddie Murray and Tony Gwynn. (Simon was a noted baseball fan.)

Dozens of spiral notebooks are also packed with not just revisions and “miscellaneous attempts at plays,” as Simon wrote in one, but drafts of speeches and tributes Simon delivered. In one case, a script for a show called “202 and 204” is interrupted by handwritten letters to cast members of “Lost in Yonkers” for opening night — plus the set designer, lighting designer, even the casting director, Horowitz said.

Horowitz said the library, once it finishes combing through the items and putting scripts in alphabetical order, plans to develop a digital tool similar to the ones they have to search other collections of work by theater professionals such as Simon’s close friends Bob Fosse and Gwen Verdon, with whom he collaborated on the musical “Sweet Charity.”

He also hopes that not just researchers, but also producers, might dive into the archives — and that some of the unproduced works might be staged and the unfinished ones perhaps completed.

“It’s so frustrating,” he said, laughing. “I desperately want to know how they end.”

This article originally appeared in The New York Times.