Community college classes
Extension Service classes
Garden society meetings such as the Master Gardener, Cal Hort., Local Bot. Garden
Local nurseries that offer weekend classes
Local CLCA
Local APLD
Local Water department
The library
Amazon, Borders and or your local independent book store
Cruise the web

I am a firm believer in the value of a more formalized training approach when it comes to landscape design. Reading and self-study are great but require a significant amount of self discipline to be of benefit and typically lack the interchange and hands-on practicum that a classroom setting offers. As do most of the online study programs.

The California community college system is excellent and very wide spread throughout the state. And many offer horticultural programs that include landscape design. As this is often a course of study undertaken by practioners in the industry who wish to increase or expand their skills - much like yourself - night classes are often available.

The link will connect you to a listing of colleges that offer this program. Just scroll down to 'Horticulture'.

If you do elect to proceed on your own, one text that I think is essential and is often used by many programs as a design textbook is Booth and Hiss: Residental Landscape Architecture; Design Process for the Private Residence.

Here is a link that might be useful: California community colleges - hort programs