An excellent, informative chapter on the lives of charioteers in ancient Rome as well as the nature of the circus and the crazed racing fans: https://www.academia.edu/1601459/Roman_Chariot-Racing_Charioteers_Factions_Spectators
No need to join the site or download the pdf; just scroll down a bit to read the chapter for free. Great stuff!
Research for this WIP chapter from Book 3. Gaius, Allerix, and Varius head to the Circus Maximus.