Books that I have read include: El Camino Real. The Golden Road: The Story of California's Spanish Mission Trail by Felix Riesengerg, Jr.(1962). Two other books are Helen White's El Camino Real de California (1956) and Edwin Corle's 1949 book The Royal Highway (El Camino Real). Charles Outland published a book that might be helpful: Stagecoaching on Camino Real Los Angeles to San Francisco 1861 - 1901.
Another publication that might be useful (but which I have not read personally) is The Romance of El Camino Real, with Authentic Kaloprints Attesting to the Period of Construction (1769-1830) , the Period of Depletion- (1835-) and Partial Preservation of the Historic California Missions (California Historical Record, Memorial No. 761)
The most extensive documentation I have seen on the original road (but focusing on the route from Mexico to San Diego) appeared in the Winter 1977 issue of the Journal of San Diego History (Volume 23, No. 1) available online at http://www.sandiegohistory.org/journal/77winter/baja.htm
You might check their sources.