The Southern Pacific:
1. "The Southern Pacific: 1901-1985," by Don Hofsommer, 1986.
https://www.amazon.com/Southern-Pacific-1901-1985-Don-Hofsommer/dp/0890962464/ref=sr_1_8?s=books&ie=UTF8&qid=1530304737&sr=1-8&refinements=p_27%3ADon+L.+Hofsommer
2. "Sunset Limited: The Southern Pacific Railroad and the Development of the American West, 1850-1930," Richard Orsi, 2005, 615 pages
https://www.ucpress.edu/book/9780520251649/sunset-limited
This is a super and very well-researched and detailed history of the SP. Top recommendation!
3. "Riding the Rails: Inside the Business of America's Railroads (Past and POResent)," by Robert D Krebs, 2018
https://www.amazon.com/Riding-Rails-Business-Americas-Railroads/dp/0253031869
Krebs was CEO of the SP, the Santa Fe, and the BNSF. An excellent read.
4. On Trainorders, all of Michael Ongerth's "Mad Dog Chronicles". He has made more than 300 posts about his career on the SP. A very rare and very interesting insiders's look at a major railroad.