My tidbits on history have been a combination of all the below. Further reading for those interested in the history should consult these books (not in any kind of order)

BOOKS

Trails of the Angeles, John W. Robinson

Mines of the East Fork, 1980. John W. Robinson

Mines of the San Gabriels, 1973. John W. Robinson

The San Gabriels I & II, 1977. John W. Robinson

The San Gabriels, 1991. John W. Robinson

Gateways to Southern California, 2005 by John W. Robinson

A Guidebook to the San Gabriel Mountains of California, 1963. Russ Leadabrand

A History of the San Gabriel Mountains, Part I-IV. Charles Clark Vernon

Early Gold Mining Days in the San Gabriel Canyon, Frank Rolfe

History of Pasadena, 1895. Hiram Alvin Reid

I Can Remember Early Pasadena by Jennie Hollingsworth Giddings

The Southern Sierras of California, 1923. Charles Francis Saunders

Afoot & Afield Los Angeles County by Jerry Schad

Historic Aircraft Wrecks of Los Angeles County by G. Pat Macha

Los Angeles Star. Various issues from California Digital Newspaper Collection

Azusa Herald, Various issues from Azusa, city website.

Trails Magazine, 1934-1939. From Department of Parks and Recreation Trails County of Los Angeles 

Los Angeles in the 1930s. The WPA Guide to The City of Angeles. Written by the Federal Writers Project of the Works Progress Administration

Images of America:

  • The Arroyo Seco by Rick Thomas
  • Azusa by Jeffrey Lawrence Cornejo Jr.
  • La Crescenta by Mike Lawler and Robert Newcombe
  • Mt. Baldy by Kimberly J. Creighton
  • Wrightwood and Big Pines by Pat Krig and Barbara Van Houten

 

WEBSITES

ADVENTURE HIKES AND CANYONEERING IN THE SAN GABRIELS

Field Guide to the San Gabriel Mountains

San Gabriel Mountains Discussion Forum

Gold Mines of Los Angeles County

Modern Hiker

Nobody Hikes in LA

Hikespeak