This study of initiatives from 1912 through July 2013 by the Secretary of State's office offers a concise history of initiatives and the initiative process. The data contained here is arranged several ways in order to accommodate various research perspectives and goals.
University of California Hastings College of the Law maintains a comprehensive, searchable database of information on California ballot initiatives and propositions from 1911 to the present. The Hastings site also offers PDF versions of ballot pamphlets from 1911 to the present.
Following each local election cycle, the California Elections Data Archive (CEDA) collects and compiles results from city, county, school district, and local ballot measure elections.
County elections officials and municipal legislative bodies are required to file a report with the Secretary of State in each odd-numbered year that contains information regarding measures placed on the ballot by the citizen initiative process.