(Beaten by Neils...)
Julian, it depends what you're after.
If you're after a spotters guide to CMPs then I suggest you track down a copy of the new book "Drive to Victory" by Clive Law (Service Publications). I picked my copy up at W&PR from Paul Meekins.
Alternatively, try and hunt down the Canadian Military Vehicle series from 1979/early 80s:
Volume 1 "Europe WW2", or (revised edition) "Canada's Fighting Vehicles"
Volume 2 "Canadian Military Vehicle Profile Series"
Volume 3 "Blueprint for Victory" by William Greg - this one is a history of the design and production of CMPs
A bit more niche is Australian Military Equipment Profiles Vol 1 (Field Artillery 1939-1945) - this has good coverage of the Australian pattern FATs, both those based on CMP running gear and the Marmon Herrington.
India also created their own local pattern FATs on CMP chassis. Here it's more a case of hunting round for information.
I think there are also 'walk around' booklets that are more aimed at the model making world.
You'll find the rich variety of CMPs endlessly absorbing (and mystifying at times).