There has been many works published on California postal history. The California market place is really 40 or so different marketplaces.
Most collectors collect California by county or by a small group of counties.
The most popular counties, like Monterey, Inyo and Mono, are as popular as any geographic area in the entire county. Some of the less collected gold rush counties are as unpopular as any geographic area in the country. In general, the market is strongest for southern material and weakest for the gold rush area. In general southern California has more interest than northern California. There does appear to have been a slight overall lessening of interest.