This is a subjective list of 5 excellent refernce works about the Great Debate in 1920. The second and third texts cited below have texts electronicly reprinted accessable from the 75th Anniversary Astronomical Debate Home Page.
Berendzen, R., Hart, R., and Seeley, D. 1976, Man Discovers the Galaxies, (Science History Publications, New York)
Curtis, H. D. 1921, The Scale of the Universe, Bull. Nat. Res. Coun. 2, 171; and Shapley, H. 1921, The Scale of the Universe, Bull. Nat. Res. Coun. 2, 194
Hoskin, M. 1976, The 'Great Debate': What Really Happened?, J. Hist. Astron., 7, 169
Smith, R. W., 1982, The Expanding Universe, Astronomy's 'Great Debate' 1900-1931, (Cambridge Univesity Press, Cambridge)
Struve, O. and Zebergs, V. 1962, Astronomy of the 20th Century, (McMillan, New York)
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