This work should be supplemented with regional or county place name guides where possible. You should also be aware that the indexes to publications of the California Division of Mines will list places not covered in this work. This includes a general history of the origin of California Counties as well as a detailed county by county discussion of boundary changes. Long out of print, it was reprinted by Valley Publishers of Fresno in 1973. This is a fine historical dictionary, published in 1978.
Entries range from a paragraph to a couple of pages for major topics.
Though new, it is already a standard source. Both of these atlases are notable for the beautiful graphics they use. They are excellent for presenting complicated material in an easily understood way. There are several good general one volume histories of California you may have available in your library. You should become familiar with the ones you have, both in the reference and circulating collections.