1. Beyond the Galaxy: The Development of Extragalactic Astronomy 1885–1965, Part 1
2. An excellent starting point on Yerkes and its telescopes is Osterbrock D., Yerkes Observatory 1892–1950: The birth, near death, and resurrection of a scientific research institution (Chicago, 1977). For Osterbrock's view on the quality of Hubble's dissertation, see p. 73. On Ritchey's 24-inch telescope, see also Osterbrock D., Pauper and prince: Ritchey, Hale, and big American telescopes (Tucson, 1993), 43–78.
3. Beyond the Big Galaxy: The Structure of the Stellar System 1900–1952