Cosmic Expansion, the fifth chapter from The Extravagant Universe: Exploding Stars, Dark Energy and the Accelerating Cosmos by Robert Kishner. Copyright © 2012 by Princeton Universtiy Press. 

Background: From Einstein to Hubble from Cosmology and Controversy: The Historical Development of Two Theories of the Universe by Helge Kragh. Copyright © 1999 by Princeton 

On the Relation Between the Expansion and the Mean Density of the Universe by Einstein, Albert and Willem de Sitter. 

Mach, Einstein and the Search for Reality by Gerald Holton. Note, to access this you will need to create a free online account with MyJSTOR. Mach、