Personally I get tried riding the same trails very often. If you just want to ride all the usual rides there is allot of info out there. If you want to find out of the way places and be more adventurous I suggest that you by USGS maps from REI or by the whole USGS map set of California on disk.
There are also other solutions out there. Investing in a handheld GPS also helps finding trailheads that are hard to find. Many hikers and bikers post tracklogs online that can be downloaded to a handheld if you have the right Topo software.
The best paper/plastic trails maps ($6-9) I have found are from Tom Harrison Maps, but they only cover parts of Cal. His map of Mt. Wilson is excellent, so far I have only found one error.
http://www.geoladders.com has good maps and directions and is adding more routes all the time.
A great site for downloading gps tracker files is http://www.singletrackmind.com I think this is free. http://www.trails.com is a fee service with maps, but they have a 15 day free trial you could check out.
If you just looking for turn-by-turn directions and how to get there, then there are a lot of other sites: http://www.ronbigelow.com/Outings/region.htm is good with pics, topo map (not easy to read, though), profile, and directions. Good books for the area include Mountain Biking Orange County, Mountain Bike! Southern California and the Falcon Guide, "Mountain Biking Southern California".
But hey, getting a little bit lost is part of the adventure, right?