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.
Rhodes, R. F., Architecture and Meaning on the Athenian Acropolis (Cambridge, 1995). A modern view of the Acropolis and its monuments.
Tanoulas, T.; Ioannidou, M.; and Moraitou, A., Study for the Restoration of the Propylaea, Volume I (Athens, 1994) Superb drawings and many careful observations from those responsible for modern work on the Propylaea.
Tanoulas, T., The Propylaea of the Athenian Acropolis during the Middle Ages, (Athens, 1997) Two volumes. In modern Greek, but includes a lengthy English summary and many drawings and photographs.
Travlos, J., Pictorial Dictionary of Ancient Athens (London, 1971). An encyclopedic approach to the monuments of Athens.
Waele, J. A. K. E. de, The Propylaia of the Akropolis in Athens: The Project of Mnesicles (Amsterdam, 1990). More of a study of planning processes than of the building.
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.
William B. Dinsmoor, William B. Dinsmoor, Jr., The Propylaia to the Athenian Akropolis II: The Classical Building Edited by Anastasia Norre Dinsmoor. Princeton, NJ: The American School of Classical Studies at Athens, 2004. This should be read with great care and in conjunction with the review, by T. Tanoulas, in the Bryn Mawr Classical Review at https://web.archive.org/web/20190805043321/http://bmcr.brynmawr.edu/2008/2008-04-17.html.. :)