These are resources which I have found useful while touring in California, and not a complete list of resources available to cycle tourists in California.
Krebs Cycle Products sells detailed maps of several regions of California. These indicate where food, water, lodging, and steep grades are. There is also (mostly accurate) information about which state parks have hiker-biker campsites. Unfortunately only a few parts California are covered.
In addition to publishing a bicycle touring magazine, the Adventure Cycling Association sells bicycle touring maps covering a route along the coast of California. These maps provide detailed information about campgrounds, motels, and bicycle shop locations for those who wish to tour along the coast. Tourists who carry cell phones will be pleased to discover that these maps include phone numbers for lodging and bicycle shops.
The US National Forest Service sells maps which show all minor roads inside national forests, and indicates which campgrounds have reliable sources of potable water. These maps are available from some stores, forest service offices, chambers of commerce, and other locations in and near the national forest. For the national forests inside California, there is now a central web page where you can find ordering information. If you plan on lighting a campfire or portable stove, please contact officials from the national forests you will be passing through, and ask whether you need a campfire permit.
The Caltrans bicycle programs unit provides free touring maps which provide provide information about grades, food, lodging, road width, and traffic conditions for California state highways. In my experience, the provided information about which towns have bicycle shops is unreliable, and is omitted in more recent printings. These maps are only available for some parts of the state, and appear to be updated about once every ten years. To obtain paper copies of these maps, you need to contact Caltrans and provide them with your postal mail address.
DeLorme sells topographic atlases. These provide extremely useful information in rural areas, but insufficient detail in urban ones. They are also too large to fit in any but the largest of panniers.
Bicycling the Pacific Coast : A Complete Route Guide Canada to Mexico, by Tom Kirkendall and Vicky Spring is occasionally referred to as the "bible." Something like 4/5 of the tourists you meet along the coast will be carrying this book. It is geared to those who wish to travel from north to south (the sensible direction, since that follows the prevailing winds) and who wish to camp in state park hiker/biker sites, and who wish to avoid the forgotten coast area.
CABO publishes its own list of California-specific touring resources. The information is particularly useful for finding out about freeway access.
In addition to fantastic scenery, most of the state parks and recreation areas in California have campgrounds, and about half have "hiker/biker sites" where you can arrive by foot or bicycle and be assured of a spot to pitch your tent without need for a reservation. Unfortunately, in most of the state the only way to find out whether a particular park or recreation area has a hiker/biker site is to inquire by telephone.
Many cycling clubs will gladly provide you with information. In general, the best way to obtain information is to ask on a club's e-mail list. See http://www.actc.org/links/clubs.html for a partial list of cycling clubs in California. The more specific the question you ask, the more useful an answer you are likely to get.