Mr Punch's History of the Great War
Lots of cartoons and explanations of them from 1914-1918 with a very readable account to put them in context of what was happeneing. Worth the £4-95.
British Trench Warfare 1917-1918
N & M reprint of an official publication.
Good read on construction and maintenance of trenches, instructions for reliefs, trench stores, wiring etc., lots of diagrams and explanations. Expensive at £18 but worth it I reckon.
I know that this was done in conjunction with IWM and an American printer but I do find the use of both defence and defense and maneuver and manouvre, sometimes on the same page a little odd.