The Santa Barbara Botanic Garden has a wellstocked Garden Shop features an outstanding selection of books and guides including the Botanic Garden's own publications. Topics include gardening, botany, landscape design, California native plants and natural history, and a cornucopia of other plant related subjects. Educators will find many resources for science education.
The Theodore Payne Foundation in Sun Valley also has a good selection of books on their online book store.
The California Native Plant Society (CNPS) publishes many excellent books on native plants, selling them through their online book store. Local chapters usually sell books at their plant sales and even at monthly meetings (check with your local chapter for upcoming events).