The Chicano Resource Center is a special collection of materials covering the history and culture of Mexican-Americans. It is housed in LA County's East Los Angeles Library.
LA County Library has also compiled a brief community history of East Los Angeles, including a community photo collection project called Foto East LA.
Check out this webpage with information about South Gate's Community History.
Tessa is LAPL's digital collection where you can find historical maps, and local photo archives. If you want to take a trip back in time, type "East Los Angeles" in the search box.