What are books are available to get an understanding of California prior to the gold-rush of 1849?
A great resource for pre-Gold Rush California is the book "Two Years Before the Mast" by Richard Henry Dana who stepped away from Harvard Law School and signed up for a tour on the merchant vessel out of Boston in 1834. The book describes the voyage around the treacherous Cape Horn under the leadership of a hard and driving captain who shows little remorse or regard for his crew as the ship is battered below the cape by the confluence of the Atlantic and Pacific oceans. Awareness of Ahab of Melville's Moby Dick cannot help but inform your mental picture of this captain. 

The ship makes its way up the coast, passing the newly independent countries of South and central America, to the northern part of Mexico, an area called Alta California. Traveling along the coast, the ship trades with the Spanish and indigenous peoples in tiny settlements and missions named San Diego, San Pedro, San Juan, Nuestra Señora La Reina de los Angeles de Porciuncula, Santa Barbara, Monterey and north to San Francisco Bay. 

Dana's detailed descriptions of the natural harborages and the tiny settlements and missions are richly described and contrast sharply with the California we see today. One is left with the singular impression of the remoteness of California to those traveling from east cost of North America or Europe.  

The crew was trading in hides and tallow, incredible to those reading from contemporary times, who consider the distance of the journey and the archaicness of the cargo. The trading goods seem altogether more ancient when considering the change the entire area would undergo in 1848 at Sutter's Mill.