For a primary source, take a look at The Legend of Joaquin Murrieta; it's a novel published in 1853 that while not being true, strictly speaking, does probably capture some of the violent nature of early American California.

For a secondary source, take a look at William Deverell's Whitewashed Adobe (University of California, 2004) I don't recall the name exactly, but The first chapter is "The Unending Mexican War," deals with the early American period in Los Angeles, and the violence deployed against Mexicans. Edit: Also take a look at Douglas Monroy, Thrown Among Strangers (University of California, 1990); it's an older work, but Monroy is an important author for the last generation of California and Mexican history. I think Elisabeth Haas might have some work of interest to you as well, but I don't see her book on the shelf. I'll see if I can find it.