History of the British Army Postal Service, Vol II, 1903-1927, Edward B Proud, Proud-Bailey Co Ltd, 1983. 312pp, hardback, illustrated with maps and photographs.
Postmarks were scrambled during WW1 so as to not reveal locations, therefore researching them can be challenging. Proud's book provides images of postmarks along with a location based on a date. Unfortunately it is not available online, Thearchives department at the Postal Museum (see above)have a copy of this book and may be able to help you with your inquiries.