Postal History of WW1

[Michael Dobbs]

I have been asked this simple question:

[color=maroon]Is there a good introductory book on the postal history of WW1?[/color]

Whilst there are books on British, American, German, Canadian, etc postal services, is there any general book which may cover all aspects of postal history during WW1 from all nations ? (which is what I believe the person making the enquiry wants).

Thanks, Mike