Flora of the Santa Ana River and Environs by Oscar F. Clarke, Danielle Svehla, Greg Ballmer and Arlee Montalvo - Very well organized, really good local botany book. Extensive grasses and weed coverage.
Wildflowers of Orange County and the Santa Ana Mountains by Robert L. Allen and Fred M. Roberts Jr. - Has very thorough coverage of most local flowers, probably the best local flower book! Have interacted with the authors, very nice people!
Foraging Southern California by Douglas Kent - Great book for knowing what commonly found plants are edible. Very well structured, with deadly/toxic plants at the beginning.