I read the WWI books by Gilbert, SLA Marshall, and Keegan about a decade ago. I enjoyed all of them as they offered quite different perspectives and focus.

Looking at them as strictly operational histories versus a more "war and society" approach, Gilbert's work falls into the latter whereas the other two are more slanted toward military although not purely operational. More than the other two he includes the views of society toward the war, notably the peace movement. I enjoyed how he interwove primary source quotations throughout his work.