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Another book that's not public domain but which can be legally downloaded.
This is a fantastic catalog to a 2012 exhibition at Madrid's Fundación Juan March. There are reproductions of dozens of works ranging from political flyers to advertisements to Upton Sinclair book jackets. The catalog has the usual introductory essays (you can download it in Spanish and English editions); writings contemporary with the work from Tschichold, Aragon, Hoch and others; and the catalog of a 1931 exhibition on the same topic.
All of the foundation's catalogs can be downloaded. Others that seem promising are on Arcimboldo, Schwitters, surrealism in the graphic arts, Malevich. The older ones appear to be Spanish-only.
