Guides
You can't tell the players without a program. For many players, you'll need several programs. Years ago I relied on one guide: the old edition of the National Audubon Society Field Guide to North American Butterflies (since improved). I now look back at some of the (mis)identifications I made, based on my limited information and my fuzzy photographs, and I laugh. Now I have a much better camera and lots of guides (and more experience) and I can usually figure out what's what thanks to the newer guides. It doesn't help that there are quite a few butterflies whose taxonomy is in question. But for the butterflies you are likely to encounter, these guides will help you put a name to what you've seen. If you're an absolute beginner (and live in Southern California) I would get two books to start: Fred Heath's book and either the Kaufman guide or Glassberg's book.
An Introduction to Southern California Butterflies, Fred Heath with photographs by Herbert Clarke, 2004. A good general introduction to our area's leps for beginners.
California Butterflies, John S. Garth and J.W. Tilden, 1985. Has some valuable information on specific locations and subspecies; I find myself using this book now as much as I ever did, though it needs to be revised.
The Butterflies of Southern California, Thomas C. Emmel and John F. Emmel, 1973. A classic, and still helpful with locations. A new book on California butterflies by the Emmels is in the works.
Butterflies and Their Favorite Flowering Plants: Anza-Borrego Desert State Park and Environs, Lynn and Gene Monroe, 2004. A thorough book on a fantastic place to look for butterflies. For each species they have usually provided a photograph of the larval food plant, which is a great idea. Information on locations within the area for each species as well.
Peterson Field Guides: Western Butterflies, Paul A. Opler with paintings by Amy Bartlett Wright, 1999.
Kaufman Focus Guides: Butterflies of North America, Jim P. Brock and Kenn Kaufman, 2003. Revised in 2006. A nicely-produced guide for identification (and a true 'field guide'). If I choose one book to put in my backpack on a hike, just in case I see something unfamiliar, this is usually it. Illustrates various forms (regional, sexually dimorphic, seasonal, etc.) which can be very helpful.
Butterflies Through Binoculars: The West. A Field Guild to the Butterflies of Western North America, Jeffrey Glassberg, 2001. For the tough identifications, it helps to have this on hand as well. Glassberg is the president of the North American Butterfly Association.
The Butterflies of North America: A Natural History and Field Guide, James A. Scott, 1986. A storehouse of information about the biology, ecology, life cycle, food plants, etc. of butterflies. The detailed text makes this a good purchase even today; few will probably use it as a 'field guide.'
Et cetera
Systematics of Western North American Butterflies, Ed. Thomas C. Emmel, 1998. From Mariposa Press in Florida. An indispensable collection of papers, many of which describe new subspecies, such as the Moss's Elfin from the San Gabriels, and the Mormon Metalmark from the sand dunes near the Los Angeles Airport (and many more). Also an important revision of the Euphilotes blues. A huge and heavy book that can be outrageously expensive - as I type this three places offer it for ca. $350 (and I've seen it for much more). Don't pay that much - it's still in print and there were two used copies at BioQuip in Rancho Dominguez for $40 in 2007. For more advanced enthusiasts.
A Catalogue of the Butterflies of the United States and Canada, Jonathan P. Pelham, 2008. Volume 40 of The Journal of Research on the Lepidoptera. This is more than an up-to-date checklist to subspecies level (which is nice in itself). For each butterfly there is a lot of information, including type locality, where the types are deposited, and where the original description of the butterfly is published. This is a big reference book (over 650 pages) packed with information. Available from BioQuip. The taxonomic information at the Butterflies of America website is copied from this book, but it is still nice to own a copy.
Lepidoptera of North America: 3. Butterflies of Kern and Tulare Counties, California, Ken Davenport, 2007. Ken has decades of experience in this region, and his detailed information on each butterfly to subspecies level is a terrific resource. No photos, just facts. After a good introduction, he provides for each butterfly date ranges for both Kern and Tulare Counties, then discusses distribution. Then he provides numerous records (location, date, collector) for every butterfly. Spiral-bound.
Flora of the Santa Ana River and Environs, Oscar F. Clarke et al., 2007. A beautifully-produced overview of the flora of the Santa Ana watershed. Clear photos of hundreds of plants and trees organized by family.
California Desert Flowers: An Introduction to Families, Genera, and Species, Sia and Emil Morhardt, 2005. A worthy companion to the previous book; this one covers the desert side of the mountains. Lots of good photography and the text is very informative.
Nabokov's Butterflies, Ed. and Annotated by Brian Boyd and Robert M. Pyle, 2000. My favorite novelist was also a dedicated and serious lepidopterist. This book contains pretty much everything he wrote involving butterflies, including correspondence. He spent many summers on long, often eventful, road trips. The chapter from his autobiography (Speak, Memory) is worth the price of admission; it may be the most famous piece of writing about butterflies. To someone like me, this is 782 pages of pure bliss. Nabokov first sparked my interest in butterflies.
Nabokov's Blues: The Scientific Odyssey of a Literary Genius, Kurt Johnson and Steve Coates, 1999. Nabokov was not merely a dabbler or enthusiast. His arrangement of the blue butterflies in the 1940s, largely ignored for decades, was revived some 50 years later after a series of discoveries in South America.
On the Wings of Checkerspots: A Model System for Population Biology, Eds. Paul R. Ehrlich and Ilkka Hanski, 2004. The intensive study of one species over four decades by academics in California and of a related butterfly in Finland yielded a mountain of information, aspects of which the 15 papers collected in this volume present. Warning: written by academics.
The Development and Evolution of Butterfly Wing Patterns, H. Frederik Nijout, 1991. Written for a well-informed (academic) audience, but fascinating nonetheless even for an amateur.