If your list includes new homeowners or first time gardeners, “Sunset Western Garden Book” or “Pat Welsh's Southern California Gardening” can't be beat and will be used as reference guides for years to come. Are they talking about digging up the lawn and putting in a natural garden?
You might consider “Gaia's Garden, A Guide to Home-Scale Permaculture” or “California Native Plants for the Garden”. Are they doing their best to grow a vegetable garden but may need a little help or inspiration? Look at “Square Foot Gardening” or “Carrots Love Tomatoes”.
But what about the experienced gardener? Do you have someone on your list that could probably write his or her own gardening book? I guarantee all gardeners love to peruse a beautiful book full of inspiring pictures. Consider two gardening books that made the Amazon list of Best Books of 2014, “The Gardener's Garden” or “The Writer's Garden”. If you are shopping for me I'll take either of the last two!