Field Guide to Butterflies of the San Francisco Bay and Sacramento Valley Regions, by Arthur M. Shapiro and Timothy D. Manolis (order this book from Amazon.com).
Butterflies through Binoculars: the West, by Jeffrey Glassberg (order this book from Amazon.com). A new favorite, this book has very good photos, plus loads of information about butterfly behavior. For example, the author advises looking for anise swallowtails on hilltops (once I read this I had a major "of course" moment, since everytime I've seen anise swallowtails, they've been on top of hills).
Common Butterflies of California, by Bob Stewart (order this book from Amazon.com). Beautiful photos, and a good source for bay area butterfly identification.
An Obsession with Butterflies, by Ahrman Apt Russell (order this book from Amazon.com). A sweet little book packed with little tidbits about butterflies and moths. I particularly enjoyed reading about butterfly behavior, including how butterflies find host plants: they look for familiar leaves, then land on contenders and "taste" the leaves with their feet.
Butterflies and Moths of North America http://www.butterfliesandmoths.org/