The Greek word προπύλαιον,propylaeon (propylaeum is theLatin version), is the union of theprefix προ-,pro-, "before, in front of", plus the plural of πύλη,pyle, "gate", meaning literally "that which is before the gates", but the word has come to mean simply "gate building".
Dinsmoor, W. B.,The Architecture of Ancient Greece (New York, 1975 – but actually a reprint of the 1950 publication). A general book on Greek architecture; dated in many areas but valuable for the Propylaea.
Dinsmoor, W. B. Jr.,The Propylaia I: The Predecessors (Princeton, 1980). A careful study of the predecessors of the Propylaea.
Eiteljorg, Harrison, II,The Entrance to the Acropolis Before Mnesicles (Dubuque, 1993). A careful study of the predecessors of the Propylaea, with very different conclusions from those of Dinsmoor above.
Lawrence, A. W.,Greek Architecture (Baltimore, 1973). A general book on Greek architecture.
Robertson, D.S.Greek and Roman Architecture' (Cambridge, 1969). A general book on Greek and Roman architecture. Available in paper, this may be the best place to begin for those with no knowledge of ancient architecture.
Travlos, J.,Pictorial Dictionary of Ancient Athens (London, 1971). An encyclopedic approach to the monuments of Athens.
The Perseus Project. An electronic resource that provides quick information, but some of the information about the Propylaea was incorrect when the site was last checked. Several good photographs of the Propylaea are available through the Perseus project.
Cartwright, Mark. "Propylaea".World History Encyclopedia, Https://Www.worldhistory.org#Organization, 24 July 2022, https://www.worldhistory.org/Propylaea/.