The Solomon R. Guggenheim Museum in New York has made nearly 100 exhibition catalogs freely available online as part of a digitized collection called “From the Archives.” Their offering of catalogs includes single-artist exhibitions as well as multi-artist exhibitions about movements or styles, and some stand alone essays published by the museum.
The Guggenheim has made other library resources available digitally in the collection Art Resources from the Mid-20th Century: Digitized Highlights from the Libraries of Hilla Rebay and Juliana Force. And the Metropolitan Museum of Art recently made a trove of digitized exhibition catalogs available in MetPublications.