Steven Jon Leiber (American, 1957–2012) was a pioneering art dealer and collector whose activities helped spread awareness that ephemera and documentation of conceptual art and other avant-garde movements were often as integral as the art itself. He and his gallery became an important resource for many scholars, curators, and others interested in the dematerialized art practices of the 1960s and 1970s.

Leiber first opened his San Francisco gallery in 1981 specializing in works on paper, before the acquisition of a small collection of artist-produced printed matter led him to the realization that ephemera and documentation related to Fluxus and other avant-garde movements of the 1960s were increasingly important forms of artistic expression in their own right. In 1987 he moved his business to the basement of his grandmother's house in the city's Marina neighborhood and renamed it Steven Leiber Basement, thus becoming one of the first experts in the nascent field of artist archives and ephemera. His reputation spread as a result of dealer catalogs he began producing in 1992 that paid homage to historic publications and multiples, including Wallace Berman's Semina journal and the exhibition catalog for Documenta V (1972) designed by Ed Ruscha.

Leiber also worked as an appraiser, assessing the papers of, among others, pop artist Claes Oldenburg; the art groups General Idea and Ant Farm; avant-garde filmmaker Jack Smith; the seminal artist journal Avalanche; and several archives now in the Research Institute's special collections, namely those of Eleanor Antin, Charles Brittin, Hal Glicksman, and Allan Kaprow. In 2001 he curated the first major exhibition of artists' ephemera, titled Extra Art: A Survey of Artists' Ephemera from 1960–99.

The archival records comprise more than 2,600 files on artists, including invitations, articles, correspondence, posters and other multiples, and ephemera. Some of the material is very rare and was often obtained by Leiber directly from the artist or from historical peers and collectors. Also included are materials related to his activities as a curator, publisher, and lecturer, such as those documenting the preparation of the Extra Art exhibition and catalog. The archive complements such foundational Research Institute collections as the papers of Jean Brown and Harald Szeemann.