On a recent afternoon, art dealer Arnold Glimcher peered down through his red-frame glasses, examining a small work on display. "This is just brilliant," he said. Mr. Glimcher has spent five decades building the Pace Gallery—one of the most powerful in New York—and orchestrating some of the biggest deals in the contemporary-art market. So a masterpiece is required for him to shower plaudits on a particular work. But in this case, his remark was not directed at seminal pieces like Roy Lichtenstein's "Girl with Ball" or Robert Rauschenberg's "Erased de Kooning Drawing," both of which hung in the next room. Instead, it came as he examined a personal note written by artist Lucas Samaras. Mr. Samaras's letter is among a collection of pieces exhumed from the gallery's archives, highlights of which Pace will put on display for the first time as part of "50 Years of Pace," an anniversary exhibition opening Friday. The archives, begun upon the gallery's founding in 1960, contain material—letters, invitations, photos and gifts in the form of sketches and drawings—from many of the artists Pace has represented, including Chuck Close, Agnes Martin, Julian Schnabel and Donald Judd. Some of the items expose back-room brokering, like a letter sent in 1973 by installation artist Louise Nevelson to painter Adolph Gottlieb as part of an effort to woo Gottlieb to the gallery's roster: "Adolph, I'm too old to beat around the bush and know you too long for that," she wrote. "Arnold thinks that you're one of the greats and would like the privilege of showing your work." Others, like a sketch by Mr. Schnabel, are small gifts bestowed on Mr. Glimcher over the course of the decades he has spent cultivating relationships with his artists. Mr. Schnabel drew the little work on a restaurant check after a dinner spent discussing his upcomng sculpture show with the dealer. The archives contains "thousand and thousands" of pieces, Mr. Glimcher said, and four full-time employees tend to it. Because the majority of items were not originally intended for public display, Mr. Glimcher sought permission from the artists or their estates before including them in this show. Though he faced no resistance, he said, there was some correspondence he chose to keep private. "Anything I was hesitant to show, I didn't," he said, alluding to correspondence in which artists confided in him regarding marital problems. "Some things were just too personal."