Hodges & Taylor provides lots of background information and is the most complete source on Arm of Service serials but does not claim to include all formation signs.
Cole is still my first port of call to identify but further research is needed before marking up a vehicle.
In terms of the BEF of 1939 / 1940 Andrew Foulkes' superbly illustrated 'Vehicle Markings of the BEF (From the GHQ Diary of 11th April 1940)' is unsurpassed but this is of course a fairly narrow interest !