Considering the mammoth size of the most recent two-parter of History Impossible, it shouldn't be shocking that there are many sources that were used in its creation. Some were extensively used and referenced, and others were mentioned only in passing. For that reason, I'll just be posting the ones that provide the greatest context to how this episode was created, in no particular order. There were far too many individual issues of Taylorology (as well as all of the primary sources) to share efficiently, but honestly, if you were as taken in by this story as I was, you'll probably fall down the Taylorology rabbit hole just as fast as I was as well. For that reason, I'll simply share the page linking to all of the official documents of case as well as the significant secondary sources.
1. Official documents concerning the William Desmond Taylor murder: http://www.taylorology.com/official.php
2. William J. Mann, Tinseltown: Murder, Morphine, and Madness at the Dawn of Hollywood, 2014
3. Robert Giroux, A Deed of Death: The Story of the Unsolved Murder of William Desmond Taylor, 1990
4. Sidney Kirkpatrick, A Cast of Killers, 1986
5. Adolph Zukor, The Public is Never Wrong, 1953
6. David Bordwell, Kristen Thompson, Film History: An Introduction, 2nd Ed., 2002
For an added literary bonus to these sources, check out Nathanael West's brilliant The Day of the Locust (1939), which gives a great feel for the decade that followed the first golden age of Hollywood, and Gore Vidal's Hollywood (1990), which contains a fictionalized account of the Taylor murder.