Although the California Republic began in 1846, it did not achieve statehood until 1850. Prior to being a state, California was the territory of various countries including Spain, Mexico, and Russia. One of the biggest draws for pioneers was the gold rush of 1848, which helped to increase California's population.
To get an idea of the transformation of California counties over time, check out the California State Association of Counties web site, http://www.csac.counties.org/default.asp?id=5, for maps of California counties and maps of county seats from 1850 to the present. The web site also includes a history of the counties and when they were formed.
The U. S. Federal Census first included the residents of California in 1850. Only one state census was done and that was in 1852. This census and an every name index is available through the Family History Library (www. familysearch.org). For those with early Los Angeles area ancestors, the 1836 Mexican census for the Los Angeles and Orange County area is available through the Family History Library.
If you are interested in a particular book or resource and want to verify its availability, check out the California Libraries Catalogue at http://www.calcat.org/. Through this site, you can either conduct a search through all of the California public libraries or through California libraries that include private, public, academic, and government resources. Once you conduct your search, you will receive relevant "hits" for books, musical scores, sound recordings, visual media (photos, VHS, DVD programs), serials and maps. You can then choose what medium you are interested in and look over the hits. A search on the keyword "Manzanar" brings up 23 archival items including a 1993 oral history interview with Brieuc Bouche where he discusses his experience as a teacher at Manzanar, the Japanese Internment Camp in the Owens Valley. Other materials include a collection of Ansel Adams' letters and ephemera having to do with Manzanar. This is a great first step to looking for what material is available on your subject of interest, whether it be a name, place, or event, and where you can locate it. This information might then be used to acquire the item through interlibrary loan.
The Online Archive of California (OAC), http://www.oac.cdlib.org/, is a collection of historical items from various California institutions. Their web site states that their collection includes 120,000 images and 50,000 pages of documents, letters, and oral histories. You can search the archive by type of resource, either images or texts, or you can conduct a "finding aid search." This search allows you to view the different collections affiliated with your topic of interest. For example, I searched on the phrase "gold rush," in the finding aids search. I was rewarded with hits from 191 collections that are available through the OAC. Images are available for printing on your home printer and information is included about copyright restrictions. This is a wonderful resource for locating pictures of events or places frequented by your ancestors.
The California State Archives located in Sacramento offers many otherwise difficult records that may help you research your ancestor, a sampling of these records include: Mental Health Records from 1856-1972 (from State Hospitals in Stockton, DeWitt, Modesto, Mendocino and Sonoma); Prison Records 1856-1972 from San Quentin, Folsom and the Youth Authority; school reports and professional and vocational licenses for architectural examiners, engineers, cosmetologists, dental examiners, medical examiners, pharmacy board, and social work examiners. They also have census records, military records, and various county records. In addition they have the Mexican Land Grant Maps 1855-1875 with an online index. The Archives have an online question form to use in corresponding with the archivists or you may visit in person. The Archives web site is at California Secretary of State - California State Archives.
The California History Room of the California State Library, http://www.library.ca.gov/html/CalHist/cal_index.cfm includes an online guide to using the resources found at the History Room, including the California state census; the California Great Register; DAR compilations; city directories; newspapers; and the Pioneer Card File. Various other indexes are available at the History Room.
Another library that is part of the California State Library is the Sutro Library. The Sutro Library, located on the campus of the San Francisco State University, is said to have the greatest genealogy collection west of Utah. While the Sutro does not have its own web site, its collections can be searched through the main catalogue of the California State Library located at http://www.library.ca.gov/.
Several California genealogy societies also include genealogical databases on their web sites. The California State Genealogical Alliance website includes a search engine for California genealogy and historical societies at http://www.csga.com/. Another resource for finding the web sites for California genealogy societies is through Census Finder at http://www.censusfinder.com/california-genealogy-society.htm.
The Southern California Genealogical Society has indexes that include Avalon Cemetery, Catalina; 1890 building permits; Fairfax High School Class of 1932; LA County Burial Permits 1870-1892; LA Times Obituary Search; and the 1896 Sierra Madre Great Register. This Society has plans for many other online indexes and their web site is a great resource. You can find them at Southern California Genealogical Society.
The Santa Clara Genealogy Historical and Genealogical Society at http://www.rootsweb.com/~cascchgs/, includes several databases including indexes for Santa Clara county histories; Index to Early Settlers File; Index to Amos Williams Funeral Home Books; Monahan Funeral Home Books Index; and the Mission City Memorial Park Index.
The San Bernardino Valley Genealogy Society has a link to the online pre-1905 death index for San Bernardino County at http://sbpl.org/genealogy.html. San Bernardino County is the largest county in the United States. San Bernardino County was created from Los Angeles County in 1853.