This book is both excellent and bad, but the excellent greatly outweighs the bad.
First, the bad: it is OLD. It is more than a decade old as I write this. A lot of development has taken place in SoCal since it was written, and I've encountered two situations where driving directions were no longer accurate. Both of them involved rides starting in Corona. I found the ride in both situations, but you need to be aware of this. If you're not familiar with the area you want to ride, I suggest checking out the info using a mapping/directions website like Google or Yahoo maps. That should be more than enough to get you to any trail.
As far as the good goes: this book has very clear trail directions and with one exception, I have never been lost following their directions (the one incident was probably my fault). By far the most useful part of the book is their trail ratings for each ride. I find the "grunt factor" to be my most valuable tool in picking a ride, as I'm still kind of new and am not capable of some of the rides with higher "grunt" ratings. Their maps are fantastic, trail rating info is fantastic, and everything else is just pretty much fantastic.
In conclusion, if you ride in OC you should own this book. I've ridden many of the trails in it and have found the info to be more than 90% accurate, even with a decade separating me from the publishing date. Cross-reference some of the driving directions and you'll have the best guide you could ask for.