When it comes a recommended reading on the Key System, you won't find any better than Harre Demoro's "Key Route -- Transbay Commuting by Train and Ferry" -- Volumes I & II, Interurban Press Specials #95 and 97. Nobody ever covered the SF-Oakland transit scene better than he. Both volumes are out of print but still readily available. There's a good likelihood of picking up one or the other volume from a vendor at any large train show.
Additionally, Vernon Sapper published a volume (again through Interurban Press) entitled "Streetcars of the Key System." Enjoyable, but skimpy on information, particularly when compared to the previous volumes.
You might also put yourself on a hunt for a video which came out a number of years ago, produced locally by KTVU, which features a wealth of vintage film covering the Key System, Sacramento Northern, Northwestern Pacific, etc. (There's even a clip of the long-gone Castro Street cablecar from San Francisco.) The title escapes me (as did my copy of the tape a couple of years back), but it, too, is readily available at any good train show. Well worth the effort.
While I'm at it, though, a recent comment you made about old movies (on the LAUPT thread) puts me in mind of a couple of other old films: "Song of the Thin Man" (1941) features a scene shot on the Bay Bridge, showing its original two-way traffic configuration on the upper deck. Also, "After the Thin Man" (1936) has a scene that was shot at the exterior of SP's 3rd & Townsend terminal in San Francisco.
Regards, Paul P.S. Another Key System photo, this time on the bridge (undated, by definitely post-1946, judging from the two-lane vehicle traffic, and shot from the cab of another train).